Question

Properties of Gases Extra Problems Name Date Extra Practice with Gas Law Problems For each of the problems below, identify the quantities that are changing (n, P, V, and/or T) and the quantities that are held constant (n, P. V, and/or T). Write the name of the applicable gas law and its two- condition equation. Label the given information (e.g. P solve the problem Vi , etc.) and then set up and If the pressure of 55 mL of an ideal gas is raised from 1.2 atm to 1.8 atm, what will its new volume be if the temperature is held constant? 1. quantities changing: quantities held constant: gas law name: gas law equation: labeled variables: Set up and solve: 2. A 250. mL sample of helium gas at 35°C is cooled until its new volume is 100. mL. temperature of the gas sample (assuming constant pressure)? What is the final quantities held constant: quantities changing gas law equation: gas law name: labeled variables: Set up and solve: Page 167 Chemistry 10 2019-2020

Answer 1

The total number of variables which can change or remain constant is 4: Pressure(P), Temperature (T), number of moles (n) and Volume (V).

By reading the question we can understand which remains constant and which changes, and choose the law accordingly.

1.

In the question, it is mentioned that the pressure is raised. So, P must change. The temperature is kept constant. It is mentioned that we must find the new volume. So, volume is the other variable which changes. It isn’t mentioned anywhere that the gas escapes or similarly. Hence, n is constant too.

Quantities changing : P, V             Quantities held constant: T,n

Gas Law name: Boyle’s Law (Pressure-Volume Law)          Equation: P₁V₁ = P₂V₂

Labelled Variables : (1 = initial state, 2 = final state)

P₁ = 1.2atm,P₂ = 1.8atm

V₁ = 55mL , V₂ = to find.

Putting in the equation :

1.2atm x 55mL = 1.8atm x V₂

Or, V₂ = 36.67mL

2.

From the question, we can again infer that T much change (since the sample is cooled), and the Volume is obviously changed. P and n are again constant.

Remember that when using the Law, the unit of T must always be in Kelvin.

Quantities changing : V,T              Quantities held constant: P,n

Gas Law name: Charles’ Law (Temperature-Volume Law)               Equation: V₁T₂ = V₂T₁

Labelled Variables : (1 = initial state, 2 = final state)

V₁ = 250mL , V₂ = 100mL

T₁= 35°C = (35+273)K = 308K. T₂=to find

Putting in the equation :

250mL x T₂ = 100mL x 308K

Or, T₂ = 123.2 K