What is the mass in grams of $0.25 \ mol$ water (in $g$)?

The number of oxygen atoms in $4.4 \ g$ of $CO_2$ is approximately ........ $\times 10^{23}$.

How many moles of oxygen gas at $STP$ are equivalent to $5.6 \ L$?

$19.7 \ kg$ of gold was recovered from a smuggler. How many atoms of gold were recovered? (Atomic mass of $Au = 197 \ g/mol$)

$A$ mixture has $18 \ g$ of water and $414 \ g$ of ethanol. The mole fraction of water in the mixture is (assume ideal behavior of the mixture):

$A$ gas mixture contains $50\%$ helium and $50\%$ methane by volume. What is the percent by weight of methane in the mixture?