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Week 4 Quiz

Problem 1

Week 4 Quiz

1. A sample of zinc metals reacts completely with excess hydrochloric acid:

Zn(s)   +   2 HCl(aq)     →     ZnCl2(aq)   +   H2(g)

If 7802 mL of gas is collected over water at 20.0°C and a pressure of 754.0 mmHg, determine the initial mass of zinc used (in grams).  Note that the vapor pressure of water at 20.0°C is 17.535 mmHg.

2. Determine the speed (uRMS) and kinetic energy of a single ammonia molecule at 25.0°C.

3. In the exosphere (the highest region of our atmosphere), helium atoms are observed to have speeds of 3530 m/s.  Based on this, estimate the temperature in this region of the atmosphere.

4. A mixture of lead sulfide, PbS, and zinc sulfide, ZnS, has a mass of 6.12 g.  This mixture was reacted with hydrochloric acid to form H2S based on the following reactions:

ZnS(s)   +   2 HCl(aq)     →     ZnCl2(aq)   +   H2S(g)

PbS(s)   +   2 HCl(aq)     →   PbCl2(aq)   +   H2S(g)

The sample reacted completely and produced 1.049 L of H2S at 23.0°C and 745 mmHg.  Determine the percentage of ZnS and PbS in the mixture.

5. Ammonia burns in the presence of a copper catalyst to form nitrogen gas.   4 NH3(g)   +   3 O2(g)    →     2 N2(g)   +   6 H2O(l) ΔH = ?1267 kJ

What is the enthalpy change to burn 35.0 g of ammonia?

6. You want to make a cup of tea which requires you to raise the temperature of 0.180 kg of water (enough for one cup of tea) from 19.0°C to 96.0°C.  Taking the specific heat of water as 4.184 J/(g °C), how much heat is required for this heating process?

7. Consider the following reaction

NH3(g)   +   CH4(g)   →     HCN(g)   + 3 H2(g)

Determine the heat of reaction for this reaction given the following data:

N2(g)   +   3 H2(g)     →     2 NH3(g)   ΔH = ?91.8 kJ

C(s)   +   2 H2(g)     →     CH4(g)     ΔH = ?74.9 kJ

H2(g)   +   2 C(s)   +   N2(g)     →     2 HCN(g) ΔH = 270.3 kJ

8. Hydrogen sulfide burns in oxygen according to the following reaction:

2 H2S(g)   +   3 O2(g)     →   2 H2O(l)   +   2 SO2(g)

Determine the enthalpy of this reaction at 25°C and 1 atm using the formation enthalpies listed in your book.

9. The heat of combustion of pentane is -3509 kJ/mol.  How many grams of pentane would need to be combusted in order to raise the temperature of 1.000 kg of water from 10.0°C to 95.0°C?

a. At what temperature does CH4 have the same RMS speed as H2 at 300K?

b. One liter of methane (CH4) requires 15 minutes to effuse through a porous barrier.  One liter of an unknown gas requires 25 minutes to effuse through the same porous barrier.  Determine the molar mass of this unknown gas.

c. When 80.0 mL of 0.300 M AgNO3 and 25.0 mL of 0.275 M Na3PO4 (both at 25.00°C) are mixed together, the temperature of the resulting solution is observed to be 33.25°C.  Assuming that volumes are additive and that the specific heat and density of any solution is the same as that of water (s=4.184 J/g °C, ρ=1.000 g/mL), calculate the amount of heat released per mole of precipitate formed.

d. Determine the enthalpy for the reaction H(g) + Br(g) → HBr(g) using the following data:

H2(g) → 2 H(g)     ΔH° = 436.4 kJ/mol

2 Br(g) → Br2(g)     ΔH° = ? 192.5 kJ/mol

H2(g)   +   Br2(g)      →    2 H(g)   ΔH° = ? 72.4 kJ/mol