Chemistry 112
Worksheet Week 8
Fetzer Gislason
1) Urea (CH4N2O), molar mass 60.0 g, reacts with sodium hypochlorite to give a mixture of gaseous
CO2 and N2.
CH4N2O(s) + 3 NaOCl(aq)

CO2(g) + N2(g) + 2 H2O(l) + 3 NaCl(aq)
A sample of urea gave 2.70 mL of gases, measured at 25oC and 745 mm Hg when treated with
excess NaOCl. What mass of urea was present in the sample?
[3.25E3 g]
2) Your task is to identify a pure substance, given its percentage composition: 40.0% carbon, 6.7%
hydrogen and the rest oxygen. A 1.428 g sample of the substance was vaporized in a 250.0 mL
flask when the temperature was 325 K and the pressure was 772 mmHg. Calculate the molar
mass of the substance and its molecular formula.
[1.50E2 g/mol, C5H10O5]
3)
Nitrogen gas is put into one of the bulbs in the apparatus below; oxygen gas is put into the other.
N2
2.0 L
1.0 atm
25C
O2
3.0 L
2.0 atm
25C
Suppose that the stopcock between the two containers is opened and the gases are allowed to mix.
What is the total pressure of the gases?
[Ptotal = 1.6 atm]
Chemistry 112
Worksheet Week 8
Fetzer Gislason
Hints:
1) In gas law problems such as this one, we use “moles of gas” in the mole/mole ratio needed for
the stoichiometry. As no particular gas is specified, add the coefficients to get the “moles of
gas.”
2) This problem is a lot of work! You have to calculate molar mass and the molecular formula.
To find molar mass, use the ideal gas law to find moles. Molar mass is g/mol. To find
molecular formula, use percent composition data to find empirical formula; then use molar mass
with the empirical formula to find molecular formula.
3) This is a very conceptual problem. You start with gases that are separated in two glass bulbs.
When the stopcock is opened, the gases mix; i.e., the system is different. Think about this when
you try to solve the problem. If you use the ideal gas law, do you have the numbers for the
variables that you need?