DGD Session #1 (Modules 4-5)
1) The percent composition of an unknown gas is 41.95% C, 22.35% O, 29.36% N, and
6.34% H. What is its empirical formula? If this unknown gas has a density of 9.43 g/L
at a temperature of 555 K and a pressure of 1.00 atm, what is its molecular formula?
2) In a 10.00 L steel container, we have 55.5 g of CO2(g), 44.4 g of N2(g), and 66.6 g of
an unknown gas. The temperature is 25.0 ◦C and the total pressure is 8.888 atm.
What is the molar mass of the unknown gas?
3) The density of a sample containing only CO2(g) is 0.666 g/L. The root-mean-square
speed (or average speed) of the CO2(g) molecules is 555.5 m/s. What is the total
pressure within this sample?
4) We do the combustion of pentane, C5H12(l) (N.B. combustion is the reaction of a
substance with O2(g) to produce CO2(g) and H2O(l)). What are the values of Q, W,
∆U, and ∆H for the combustion of 1.000 mol of pentane under a constant pressure
of 1.000 atm and a temperature of 25.0 ◦C? What would the value of Q be if we did
the combustion of 1.777 mol of pentane at constant volume at 25.0 ◦C?
Data
• ∆H◦ f (C5H12, l) =-173.5 kJ mol−1
• ∆H◦ f (CO2, g) =-393.5 kJ mol−1
• ∆H◦ f (H2O, l) =-285.8 kJ mol−1
5) We place 100.0 g of a metal at 100.00 ◦C in 222.2 g of water at 20.00 ◦C. The water is
in a beaker that is also at 20.00 ◦C. The specific heat of water is 4.184 J K−1 g −1 and
that of the metal is 0.4444 J K−1 g −1. The heat capacity of the beaker is 1.222 kJ K−1.
What is the final temperature of the metal, water, and beaker?
6) Calculate the value of ∆H for the reaction
CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g)
with the aid of the following enthalpies of reaction:
C(s) + O2(g) → CO2(g) ∆H=-393.5 kJmol−1
S(s) + O2(g) → SO2(g) ∆H =-296.8 kJmol−1
C(s) +2 S(s) → CS2(l) ∆H =+87.9 kJmol−1
All of the data is at 25 ◦C.