Which of the following species has the highest mass?

Identify the compound formed from elements $X, Y, Z$ having oxidation states $+2, +5, -2$ respectively.

Which of the following contain the same number of atoms?
$A$. $2 \text{ g}$ of $O_2$ gas
$B$. $4 \text{ g}$ of $SO_2$ gas
$C$. $1400 \text{ mL}$ of $O_2$ at $STP$
$D$. $0.05 \text{ L}$ of $He$ at $STP$
$E$. $0.0625 \text{ mol}$ of $H_2$ gas
Choose the correct answer from the options given below:

Calculate the molarity of each of the following solutions: $(a)$ $30 \, g$ of $Co(NO_3)_2 \cdot 6H_2O$ in $4.3 \, L$ of solution $(b)$ $30 \, mL$ of $0.5 \, M \, H_2SO_4$ diluted to $500 \, mL$.

Substance $A_2B_{(g)}$ can undergo decomposition to form two sets of products. If the molar ratio of $A_{2(g)}$ to $A_{(g)}$ is $5 : 3$ in a set of product gases,then the energy involved in the decomposition of $1 \, \text{mole}$ of $A_2B_{(g)}$ is:
$A_2B_{(g)} \to A_{2(g)} + B_{(g)}; \Delta H^o = 40 \, \text{kJ/mol}$
$A_2B_{(g)} \to A_{(g)} + AB_{(g)}; \Delta H^o = 50 \, \text{kJ/mol}$

$K_2CS_3$ can be called potassium: