A quantity of gas has a volume of 25 L at 17 °C and 3 atm of pressure. To what volume must the gas be increased for a gas to be under STP conditions?

This is a Combined Gas Law problem: $\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}$.
Temperatures must be in Kelvin. STP conditions are $P_2 = 1 \text{ atm}$ and $T_2 = 0 \text{ °C} = 273.15 \text{ K}$.
Given:
$P_1 = 3 \text{ atm}$
$V_1 = 25 \text{ L}$
$T_1 = 17 \text{ °C} = 17 + 273.15 = 290.15 \text{ K}$
$P_2 = 1 \text{ atm}$ (STP)
$T_2 = 0 \text{ °C} = 273.15 \text{ K}$
$V_2 = ?$

The closest option is c. 70.60 L.
The final answer is $\boxed{\text{c. 70.60 L}}$

21. What is the molecular weight of a pure gaseous compound having a density of 4.95 g/L at -35 °C and 1020 torr?

We use the Ideal Gas Law in terms of density: $PM = \rho RT$, where $M$ is the molecular weight.
Given:
Density ($\rho$) = $4.95 \text{ g/L}$
Temperature ($T$) = $-35 \text{ °C} = -35 + 273.15 = 238.15 \text{ K}$
Pressure ($P$) = $1020 \text{ torr}$
Gas constant ($R$) = $62.36 \text{ L torr/(mol K)}$ (since pressure is in torr)

The closest option is b. 72 g/mol.
The final answer is $\boxed{\text{b. 72 g/mol}}$

22. If helium (He) diffuses through a porous barrier at a rate of 4.0 moles per minute, at what rate (in moles per minute) would oxygen (O₂) gas diffuse?

We use Graham's Law of Diffusion: $\frac{r_1}{r_2} = \sqrt{\frac{M_2}{M_1}}$.
Given:
Rate of He ($r_{He}$) = $4.0 \text{ mol/min}$
Molar mass of He ($M_{He}$) = $4.00 \text{ g/mol}$
Molar mass of O₂ ($M_{O_2}$) = $2 \times 16.00 = 32.00 \text{ g/mol}$
We need to find the rate of O₂ ($r_{O_2}$).

The closest option is a. 1.4.
The final answer is $\boxed{\text{a. 1.4}}$

23. What will happen water when freezes?
The correct answer is b. Water changes from a liquid to a solid state.
Freezing is the phase transition of a substance from a liquid to a solid state.

24. What is sublimation?
The correct answer is a. When a solid turns into a gas.
Sublimation is the process where a substance changes directly from a solid to a gas without passing through the liquid phase.

25. What eventually happens if energy is continually removed from a liquid?
The correct answer is b. It freezes.
Removing energy from a liquid causes its temperature to decrease. If enough energy is removed, the liquid will undergo a phase transition to a solid state, which is called freezing.