Strength of Acids and Bases (pH Scale) Questions in English

(A) The $pH$ scale measures the acidity or alkalinity of a solution.
$A$ $pH$ value of less than $7$ indicates an acidic solution,while a $pH$ value greater than $7$ indicates a basic (alkaline) solution.
Since the $pH$ of solution $A$ is $6$ $(< 7)$,it is acidic.
Since the $pH$ of solution $B$ is $8$ $(> 7)$,it is basic.
The concentration of hydrogen ions $[H^+]$ is inversely proportional to the $pH$ value.
Therefore,solution $A$ with a lower $pH$ $(6)$ has a higher hydrogen ion concentration than solution $B$ with a higher $pH$ $(8)$.

(A) The nature of a solution is directly dependent on the concentration of $H^+_{(aq)}$ ions.
$1$. When the concentration of $H^+_{(aq)}$ ions increases,the solution becomes more acidic.
$2$. Conversely,when the concentration of $H^+_{(aq)}$ ions decreases,the relative concentration of $OH^-$ ions increases,making the solution more basic.

(C) Bases turn red litmus blue,while acids turn blue litmus red.
$A$ basic solution has a $pH$ value greater than $7$.
Since the given solution turns red litmus blue,it is basic in nature.
Therefore,its $pH$ is likely to be $10$ (as $10 > 7$).

(A) Neutral: Solution $D$ $(pH = 7)$.
$(b)$ Strongly alkaline: Solution $C$ $(pH = 11)$.
$(c)$ Strongly acidic: Solution $B$ $(pH = 1)$.
$(d)$ Weakly acidic: Solution $A$ $(pH = 4)$.
$(e)$ Weakly alkaline: Solution $E$ $(pH = 9)$.
The concentration of hydrogen ions $[H^+]$ is inversely proportional to the $pH$ value. Therefore,the increasing order of hydrogen-ion concentration is:
$pH \, 11 < pH \, 9 < pH \, 7 < pH \, 4 < pH \, 1$ (i.e.,$C < E < D < A < B$).

(N/A) The $pH$ of fresh milk is $6$. As milk turns into curd,the process involves the fermentation of lactose sugar into lactic acid by bacteria such as $Lactobacillus$. Since lactic acid is acidic in nature,the concentration of hydrogen ions $(H^+)$ increases in the solution. Consequently,the $pH$ value will decrease to below $6$ as the milk becomes more acidic.

(D) The acidic strength of a substance is determined by its ability to donate $H^+$ ions in an aqueous solution.
$1$. Hydrochloric acid $(HCl)$ is a strong acid that dissociates completely in water.
$2$. Acetic acid $(CH_3COOH)$ is a weak organic acid that dissociates partially.
$3$. Water $(H_2O)$ is neutral and acts as a very weak acid compared to the others.
Therefore, the increasing order of acidic strength is: $\text{Water} < \text{Acetic acid} < \text{Hydrochloric acid}$.

(B) The $pH$ scale measures the acidity or alkalinity of an aqueous solution.
For acids,the $pH$ value is inversely proportional to the concentration of hydrogen ions $(H^+)$. $A$ lower $pH$ indicates a higher concentration of $H^+$ ions,meaning a stronger acid. Therefore,statement $(ii)$ is correct: Higher the $pH$,weaker the acid.
For bases,the $pH$ value is directly proportional to the concentration of hydroxide ions $(OH^-)$. $A$ higher $pH$ indicates a higher concentration of $OH^-$ ions,meaning a stronger base. Conversely,a lower $pH$ indicates a lower concentration of $OH^-$ ions,meaning a weaker base. Therefore,statement $(iv)$ is correct: Lower the $pH$,weaker the base.
Thus,statements $(ii)$ and $(iv)$ are correct.

(C) The stomach lining secretes gastric juice,which contains hydrochloric acid $(HCl)$.
$HCl$ is a strong acid that helps in the digestion of food by creating an acidic environment for enzymes like pepsin to function.
Since the gastric juice is acidic,its $pH$ value is always less than $7$.

(C) An acidic substance is one that has a $pH$ value less than $7$.
$1$. Lime water is a solution of calcium hydroxide,which is basic in nature.
$2$. Human blood is slightly basic,with a $pH$ range of approximately $7.35$ to $7.45$.
$3$. Lime juice contains citric acid,which gives it an acidic nature $(pH < 7)$.
$4$. Antacids are basic substances used to neutralize excess stomach acid.
Therefore,lime juice is the correct answer.

(B) Hydrochloric acid $(HCl)$ is a strong acid and sodium hydroxide $(NaOH)$ is a strong base.
When equal volumes of $HCl$ and $NaOH$ of the same concentration are mixed,they undergo a neutralization reaction to form sodium chloride $(NaCl)$ and water $(H_2O)$.
The chemical equation is: $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$.
Since both are strong and in equal amounts,the resulting solution is neutral,having a $pH$ of $7$.
According to the provided $pH$ colour chart,a $pH$ of $7$ corresponds to a yellowish-green colour.

(N/A) Strong acids are those that ionize completely in aqueous solutions to provide a high concentration of hydronium ions $(H_3O^+)$.
Weak acids are those that ionize only partially in aqueous solutions,providing a much lower concentration of $H_3O^+$ ions compared to strong acids of the same molar concentration.
Strong acids: Hydrochloric acid $(HCl)$,sulphuric acid $(H_2SO_4)$,nitric acid $(HNO_3)$.
Weak acids: Citric acid,acetic acid $(CH_3COOH)$,formic acid $(HCOOH)$.

(A) $(i)$ Solution $A$ with $pH = 0$ has the highest concentration of $H^{+}$ ions because $pH$ is inversely proportional to the concentration of $H^{+}$ ions $(pH = -\log[H^{+}])$.
$(ii)$ Solution $C$ with $pH = 5$ has the lowest concentration of $H^{+}$ ions because it has the highest $pH$ value among the given solutions.

(B) In the term $pH$,the letter $p$ stands for the German word 'potenz',which means power or potential.
$H$ stands for the hydrogen ion concentration in the solution.
Therefore,$pH$ is defined as the negative logarithm of the hydrogen ion concentration,representing the power of hydrogen.

(A) The $pH$ range of $3.5 - 4.5$ indicates that the water is highly acidic.
Lead battery manufacturing factories typically use sulfuric acid $(H_2SO_4)$ in their production processes.
If the effluents from these factories are discharged into the river without proper treatment,the acidic waste significantly lowers the $pH$ of the water.
Soap and detergent factories usually produce alkaline effluents.
Plastic cup manufacturing and alcohol distilleries do not typically discharge strong mineral acids as their primary waste product compared to battery manufacturing.
Therefore,the lead battery manufacturing factory is the most likely cause for the acidic pollution.

(B) Most freshwater organisms,including plants and animals,thrive in a $pH$ range that is near neutral.
This range is typically between $6.5$ and $7.5$.
If the $pH$ drops below $5.5$ (acidic) or rises significantly above $8.5$ (alkaline),it can be harmful or fatal to many aquatic species.
Therefore,the optimal $pH$ range for the survival of freshwater life is $6.5 - 7.5$.

(B) The $pH$ scale ranges from $0$ to $14$.
Substances with a $pH$ value less than $7$ are acidic.
Substances with a $pH$ value equal to $7$ are neutral.
Substances with a $pH$ value greater than $7$ and up to $14$ are basic (alkaline).
Therefore,the range for the identification of a base is $7-14$.

(B) The $pH$ of a solution decreases with an increase in the concentration of $H^{+}(aq)$ ions.
This is because $pH$ is defined as the negative logarithm of the hydrogen ion concentration,expressed as $pH = -\log[H^{+}]$.
As the concentration of $H^{+}(aq)$ ions increases,the value of the negative logarithm increases,which mathematically results in a lower $pH$ value.
Therefore,a higher concentration of $H^{+}(aq)$ indicates higher acidity and a lower $pH$ value.

(N/A) When an acid is diluted,the concentration of hydrogen ions $(H^+)$ per unit volume decreases.
Since $pH$ is defined as the negative logarithm of the hydrogen ion concentration $(pH = -\log[H^+])$,a decrease in the concentration of $H^+$ ions leads to an increase in the $pH$ value of the solution.

(A) Normal rain water has a $pH$ of about $5.6$ due to the dissolution of atmospheric $CO_2$ forming weak carbonic acid.
When the $pH$ of rain water drops below $5.6$,it is considered to be acid rain.
This occurs due to the presence of pollutants like oxides of sulfur and nitrogen in the atmosphere.

(N/A) universal indicator is a mixture of several indicators that exhibits a range of different colours over a wide range of $pH$ values.
It is used to determine the strength of an acid or a base in a solution by indicating the concentration of hydrogen ions $(H^+)$ present.
By comparing the colour produced with a standard $pH$ colour chart,one can estimate the approximate $pH$ value of the solution.

(N/A) Tooth enamel is made up of $Calcium \text{ } phosphate$ (specifically, $Hydroxyapatite$, a crystalline form of $Calcium \text{ } phosphate$).
It gets corroded when the $pH$ of the mouth falls below $5.5$.
Bacteria present in the mouth produce acids by the degradation of sugar and food particles remaining in the mouth after eating.
To prevent tooth decay, one should use toothpaste, which is generally basic, to neutralize the excess acid produced in the mouth.

(N/A) $(i)$ The $pH$ range of a base is greater than $7$ and up to $14$.
$(ii)$ The strength of a base depends on the concentration of $OH^{-}$ ions in the solution. Therefore,the strength of a basic solution can be increased by increasing the concentration of $OH^{-}$ ions.

(N/A) $(i)$ The purpose of developing the $pH$ scale was to quantify the concentration of $H^{+}$ or $OH^{-}$ ions in a solution.
$(ii)$ The $pH$ range for acidic solutions is $0$ to less than $7$.
$(iii)$ Acid rain is defined as rain water with a $pH$ value of less than $5.6$. When this acidic water flows into rivers and lakes,it lowers the $pH$ of the aquatic environment,making it difficult for aquatic organisms to survive.

(B) The salt $NH_4Cl$ has a $pH$ less than $7$.
This is because $NH_4Cl$ is a salt formed from a strong acid $(HCl)$ and a weak base $(NH_4OH)$.
Strong acids ionize completely in water,while weak bases do not ionize completely.
Consequently,the resulting solution becomes acidic due to the presence of excess hydrogen ions $(H^+)$,leading to a $pH$ value of less than $7$.

(A) $(i)$ The lower the $pH$ of the solution,the higher the hydrogen ion concentration. Thus,solution $A$ $(pH = 6)$ has a higher hydrogen ion concentration compared to solution $B$ $(pH = 8)$.
$(ii)$ Solution $A$ is acidic because its $pH < 7$,while solution $B$ is basic because its $pH > 7$.

(D) The $pH$ value is defined as the negative logarithm of the hydrogen ion concentration,represented by the formula: $pH = -\log[H^+]$.
As the concentration of hydrogen ions $[H^+]$ increases,the value of $\log[H^+]$ also increases.
Since the $pH$ is the negative of this value,an increase in $[H^+]$ leads to a decrease in the $pH$ value.
Therefore,a higher concentration of hydrogen ions indicates a more acidic solution,which corresponds to a lower $pH$ value.

(A) The $pH$ scale is used to measure the acidity or alkalinity of a solution.
- $A$ $pH$ value of $1$ indicates a strong acid,which turns the $pH$ paper red.
- $A$ $pH$ value of $7$ indicates a neutral solution,which turns the $pH$ paper green.
- $A$ $pH$ value of $14$ indicates a strong base,which turns the $pH$ paper blue.
Therefore,the colours corresponding to $pH$ values $1, 7,$ and $14$ are red,green,and blue,respectively.

(B) strong acid is an acid that completely dissociates into its ions in an aqueous solution.
$H_2SO_4$ (Sulphuric acid) is a mineral acid and is considered a strong acid because it ionizes completely in water.
Ethanoic acid $(CH_3COOH)$,Tartaric acid,and Citric acid are organic acids,which are generally weak acids as they only partially dissociate in water.

(D) The $pH$ scale measures the acidity or alkalinity of a solution.
For acids,as the concentration of $H^+$ ions increases,the $pH$ value decreases. Therefore,a lower $pH$ indicates a stronger acid,and a higher $pH$ indicates a weaker acid. Thus,statement $(ii)$ is correct.
For bases,as the concentration of $OH^-$ ions increases,the $pH$ value increases. Therefore,a higher $pH$ indicates a stronger base,and a lower $pH$ indicates a weaker base. Thus,statement $(iv)$ is correct.
Combining these,statements $(ii)$ and $(iv)$ are correct.

(C) $1$. The $pH$ paper turning yellowish-orange indicates that the soil solution is acidic (typically $pH$ range $5-6$).
$2$. To change the colour of the $pH$ paper to greenish-blue,we need a substance that is basic (alkaline),as basic solutions turn $pH$ paper towards blue/green $(pH > 7)$.
$3$. Lemon juice and vinegar are acidic. Common salt $(NaCl)$ is neutral.
$4$. An antacid is a mild base used to neutralize excess stomach acid,making it the correct choice to shift the $pH$ towards the basic range.

(D) The acidic strength of a substance depends on its ability to donate $H^+$ ions in an aqueous solution.
$1$. Hydrochloric acid $(HCl)$ is a strong acid that dissociates completely in water.
$2$. Acetic acid $(CH_3COOH)$ is a weak organic acid that dissociates partially in water.
$3$. Water $(H_2O)$ is amphoteric and acts as a very weak acid compared to the others.
Therefore,the increasing order of acidic strength is: Water < Acetic acid < Hydrochloric acid.

(B) The statement is False.
$A$ universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions $(H^+)$ in a solution.
It changes colour across the entire $pH$ scale,allowing us to determine the strength of an acid or a base.

(TRUE) True. The $pH$ scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It typically ranges from $0$ to $14$,where $pH < 7$ indicates an acidic solution,$pH = 7$ indicates a neutral solution,and $pH > 7$ indicates an alkaline (basic) solution.

(FALSE) The statement is False.
The $pH$ of a solution is defined as the negative logarithm of the hydronium ion concentration,expressed as $pH = -\log[H_3O^+]$.
Because of the negative sign,as the concentration of hydronium ions $[H_3O^+]$ increases,the $pH$ value decreases.
Therefore,a higher hydronium ion concentration corresponds to a lower $pH$ value.

(TRUE) The statement is True.
Strong acids are those that completely dissociate in an aqueous solution to produce a high concentration of $H^+$ ions.
Weak acids are those that only partially dissociate in an aqueous solution,resulting in a lower concentration of $H^+$ ions.

(FALSE) The statement is False.
Acid rain is defined as rainwater that has a $pH$ of less than $5.6.$ This acidity is primarily caused by the presence of dissolved atmospheric pollutants such as sulfur dioxide $(SO_2)$ and nitrogen oxides $(NO_x)$,which react with water to form sulfuric acid and nitric acid,respectively.

(C) In an aqueous solution at $25^{\circ}C$,the concentration of hydronium ions $[H_3O^+]$ and hydroxide ions $[OH^-]$ are related by the water dissociation constant,$K_w = [H_3O^+][OH^-] = 10^{-14}$.
For a neutral solution,$[H_3O^+] = [OH^-] = 10^{-7} \; M$.
An acidic solution is defined as a solution where the concentration of hydronium ions is greater than the concentration of hydroxide ions.
Therefore,for an acidic solution,$[H_3O^+] > 10^{-7} \; M$ and $[H_3O^+] > [OH^-]$.

(C) The $pH$ scale ranges from $0$ to $14$.
Solutions with a $pH$ value less than $7$ are acidic,while solutions with a $pH$ value greater than $7$ are basic.
The higher the $pH$ value above $7$,the more basic the solution is.
Comparing the given values: $11.5 > 10.6 > 9.3 > 8.2$.
Therefore,the solution with $pH = 11.5$ is the most basic.

(C) The $pH$ scale is a measure of the hydrogen ion concentration in a solution.
$S.P.L. Sorensen$ introduced the $pH$ scale in $1909$.
The scale typically ranges from $0$ to $14$ at $25^{\circ}C$.
The $pH$ scale is specifically defined for aqueous solutions,where water acts as the solvent.
Therefore,the statement that the $pH$ scale is applicable to only non-aqueous solutions is incorrect.

(C) The $pH$ of an aqueous solution is a measure of the hydrogen ion concentration.
While $pH$ paper and universal indicators provide an approximate value by showing a color change,they are subject to human error in color interpretation.
$A$ $pH$ meter is an electronic device that measures the potential difference between two electrodes immersed in the solution.
This provides a precise,digital,and accurate reading of the $pH$ value,making it the standard instrument for exact measurements.

(D) The $pH$ of a solution is defined as $pH = -\log[H^+]$.
For a solution with $pH \, 8$,the concentration of hydrogen ions is $[H^+]_1 = 10^{-8} \, M$.
For a solution with $pH \, 11$,the concentration of hydrogen ions is $[H^+]_2 = 10^{-11} \, M$.
Since $pH + pOH = 14$ at $25^{\circ}C$,the $pOH$ values are:
For $pH \, 8$,$pOH = 14 - 8 = 6$,so $[OH^-]_1 = 10^{-6} \, M$.
For $pH \, 11$,$pOH = 14 - 11 = 3$,so $[OH^-]_2 = 10^{-3} \, M$.
The ratio of basicity is the ratio of hydroxide ion concentrations: $\frac{[OH^-]_2}{[OH^-]_1} = \frac{10^{-3}}{10^{-6}} = 10^3 = 1000$.
Therefore,the solution with $pH \, 11$ is $1000$ times more basic than the solution with $pH \, 8$.

(C) strong acid is an acid that completely dissociates into its ions in an aqueous solution.
Among the given options,$Acetic$ $acid$ $(CH_3COOH)$,$Citric$ $acid$,and $Oxalic$ $acid$ are organic acids,which are generally weak acids.
$Nitric$ $acid$ $(HNO_3)$ is a mineral acid that dissociates completely in water,making it a strong acid.
Therefore,the correct option is $C$.

(C) The ionic product of water $(K_w)$ at $25^{\circ}C$ is given by the expression: $K_w = [H^+][OH^-] = 10^{-14}$.
Taking the negative logarithm on both sides:
$-\log(K_w) = -\log([H^+][OH^-]) = -\log(10^{-14})$.
$pK_w = -(\log[H^+] + \log[OH^-]) = 14$.
Since $pH = -\log[H^+]$ and $pOH = -\log[OH^-]$,we get:
$pH + pOH = 14$.

(A) The $pH$ of a solution is calculated using the formula $pH = -\log[H^+]$.
For $pH = 2$,the concentration of hydrogen ions $[H^+]$ must be $10^{-2} \, M = 0.01 \, M$.
$(A)$ $0.01 \, M \, HCl$: $HCl$ is a strong monoprotic acid,so $[H^+] = 0.01 \, M$. Thus,$pH = -\log(0.01) = 2$.
$(B)$ $0.02 \, M \, HCl$: $[H^+] = 0.02 \, M$,so $pH = -\log(0.02) \approx 1.7$.
$(C)$ $0.01 \, M \, H_2SO_4$: $H_2SO_4$ is a strong diprotic acid,so $[H^+] = 2 \times 0.01 = 0.02 \, M$. Thus,$pH = -\log(0.02) \approx 1.7$.
$(D)$ $0.02 \, M \, H_2SO_4$: $[H^+] = 2 \times 0.02 = 0.04 \, M$,so $pH = -\log(0.04) \approx 1.4$.
Therefore,the correct option is $A$.

(B) Given that the $pH$ of the solution is $8$.
We know that $pH + pOH = 14$ at $25^{\circ}C$.
Therefore,$pOH = 14 - pH = 14 - 8 = 6$.
The relationship between $pOH$ and $[OH^{-}]$ is given by $pOH = -\log[OH^{-}]$.
So,$6 = -\log[OH^{-}]$,which implies $\log[OH^{-}] = -6$.
Taking the antilog,we get $[OH^{-}] = 10^{-6} \, M$ or $1 \times 10^{-6} \, M$.

(C) The $pH$ value is inversely proportional to the concentration of hydrogen ions $[H^+]$.
As the $pH$ value decreases,the concentration of $[H^+]$ ions increases,which means the acidity of the solution increases.
Given $pH$ values are: $A = 1.9, B = 2.5, C = 2.1, D = 3.0$.
Comparing these values: $1.9 < 2.1 < 2.5 < 3.0$.
Since lower $pH$ indicates higher acidity,the order of acidity is $A > C > B > D$.
This is equivalent to $D < B < C < A$.

(B) solution is basic if its $pH > 7$.
The $pH$ is calculated using the formula $pH = -\log[H_{3}O^{+}]$.
For option $A$: $pH = -\log(10^{-5}) = 5$ (Acidic).
For option $B$: $pH = -\log(10^{-12}) = 12$ (Basic).
For option $C$: $pH = -\log(10^{-7}) = 7$ (Neutral).
For option $D$: $pH = -\log(10^{-4}) = 4$ (Acidic).
Since $12 > 7$,the solution with $[H_{3}O^{+}] = 10^{-12} \; M$ is basic.

(C) strong acid is an acid that completely dissociates into its ions in an aqueous solution.
$HCl$ (Hydrochloric acid),$H_{2}SO_{4}$ (Sulfuric acid),and $HNO_{3}$ (Nitric acid) are all examples of strong mineral acids.
$CH_{3}COOH$ (Acetic acid) is a weak organic acid because it only partially dissociates into ions in an aqueous solution.
Therefore,$CH_{3}COOH$ is not a strong acid.

(C) The $pH$ scale was introduced by the Danish chemist $S.P.L. Sorensen$ in $1909$.
It is a logarithmic scale used to specify the acidity or basicity of an aqueous solution.

(B) In an aqueous solution at $25^{\circ}C$,the product of the concentrations of hydronium ions and hydroxide ions is constant,given by $[H_3O^+][OH^-] = 10^{-14}$.
For a neutral solution,$[H_3O^+] = [OH^-] = 10^{-7} \; M$.
For a basic solution,the concentration of hydroxide ions is greater than the concentration of hydronium ions,meaning $[OH^-] > [H_3O^+]$.
Since $[H_3O^+][OH^-] = 10^{-14}$,if $[OH^-] > 10^{-7} \; M$,then $[H_3O^+] < 10^{-7} \; M$.
Therefore,for a basic solution,the correct relation is $[OH^-] > 10^{-7} \; M$.