# Measuring liquids and gases

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions and Stoichiometry

### HSC Chemistry Syllabus

• Relate stoichiometry to the law of conservation of mass in chemical reactions by investigating:
• Balancing chemical equations (ACSCH039)
• Solving problems regarding mass changes in chemical reactions (ACSCH046)

### Measuring Liquids in Chemical Reactions

The density of a liquid is commonly expressed in the unit $$\text{g/mL}$$. Given both the density and volume of a liquid, its mass can be determined using the relation:

\begin{equation}
\text{mass} = \text{volume} \times \text{density}
\end{equation}

Calculate the mass of 30 mL of methanol, given its density as $$0.79 \, \text{g/mL}$$.

\begin{align*}
\text{Mass} &= \text{volume} \times \text{density} \\
\text{Mass} &= 30 \, \text{mL} \times 0.79 \, \text{g/mL} \\
\text{Mass} &= 23.7 \, \text{g}
\end{align*}

### Measuring Gases in Chemical Reactions

Volume is the preferred metric for quantifying the amount of gas in a chemical reaction. This preference arises because, unlike mass, volume isn't always conserved in chemical reactions. Thus, volume and density can assist in determining the mass of gaseous reactants.

Differing from liquids, gas particles are fewer per unit volume. Therefore, the density of a gas is typically expressed as $$\text{g/L}$$. Importantly, the density of a gas is contingent upon both temperature and pressure.

Particle theory elucidates why gas densities are temperature-sensitive. Gases comprise widely spaced particles that can be compressed easily. To understand the relationship between the amounts of gas (in moles, a topic we'll delve into in Section 2) and volumes, we resort to various gas laws.