A syringe holds 75 mL of air on a cold morning, when air has a density of 1.3 kg/m³…

Step 1. (a) The same particles fill less space, so they come closer together. This is possible because a gas has large spaces between its particles.

Step 2. (b) The mass is the same and the volume is $\dfrac{75\ \text{mL}}{25\ \text{mL}} = 3$ times smaller, so the density is $3$ times larger: $1.3\ \text{kg/m}^3 \times 3 = 3.9\ \text{kg/m}^3$.

Step 3. (c) $75\ \text{mL} = 75 \times 10^{-6}\ \text{m}^3$. Mass $= 1.3\ \text{kg/m}^3 \times 75 \times 10^{-6}\ \text{m}^3 = 9.75 \times 10^{-5}\ \text{kg} = 0.0975\ \text{g}$.