5 Steps to a 5 AP Chemistry 2019

112 ❯ STEP 4. Review the Knowledge You Need to Score High

grams of KClO 3 are present. You take the mixture and heat it. The KCIO 3 decomposes

according to the equation:

2 KClO 3 (s) → 2 KCl(s) + 3 O 2 (g)

The oxygen gas that is formed is collected by displacement of water. It occupies a

volume of 542 mL at 27°C. The atmospheric pressure is 755.0 torr. The vapor pressure of

water at 27°C is 26.7 torr.

First, you need to determine the pressure of just the oxygen gas. It was collected over

water, so the total pressure of 755.0 torr is the sum of the partial pressures of the oxygen

and the water vapor:

PPTotalO=+ 22 PHO(Dalton’slaw)

The partial pressure of water vapor at 27°C is 26.7 torr, so the partial pressure of the

oxygen can be calculated by:

PPOT 22 =−otal PHO=−755.0torr 26.7torr=728.3torr

At this point, you have 542 mL of oxygen gas at 728.3 torr and 300. K (27°C + 273).

From this data, you can use the ideal gas equation to calculate the number of moles of

oxygen gas produced:

PV = nRT

PV/RT = n

You will need to convert the pressure from torr to atm:

(728.3 torr) × (1 atm/760.0 torr) = 0.9583 atm

and express the volume in liters: 542 mL = 0.542 L

Now you can substitute the quantities into the ideal gas equation:

(0.9583 atm)(0.542 L)/(0.0821 L ·^ atm/K ·^ mol)(300. K) = n

0.02110 mol O 2 = n

Now you can use the reaction stoichiometry to convert from moles O 2 to moles KClO 3

and then to grams KClO 3 :

















(0.02110molO ) =

2mol KClO

3mol O

122.55gKClO

1mol KClO

2 3 1.723gKClO

2

3

3

3

Non-Ideal Gases

We have been considering ideal gases, that is, gases that obey the postulates of the Kinetic

Molecular Theory. But remember—a couple of those postulates were on shaky ground.

The volume of the gas molecules was negligible, and there were no attractive forces between

the gas particles. Many times approximations are fine and the ideal gas equation works

well. But, it would be nice to have a more accurate model for doing extremely precise work

or when a gas exhibits a relatively large attractive force. In 1873, Johannes van der Waals

introduced a modification of the ideal gas equation that attempted to take into account

the volume and attractive forces of real gases by introducing two constants—a and b—into

the ideal gas equation. Van der Waals realized that the actual volume of the gas is less than the

ideal gas because gas molecules have a finite volume. He also realized that the more moles of

gas present, the greater the real volume. He compensated for the volume of the gas particles

mathematically with:

corrected volume = V – nb