Ch. 10 gas          test     [55 points]            AP Chem

Show your work in an organized manner, include relevant equation(s) / formula(s), proper units in your work / answer, and the proper number of significant figures in yoru answer.

1. Determination of the molar mass of a volatile liquid. [15 points}

a. derive the equation to determine the molar mass of a gas in your recent lab.

b. error analysis: what is the effect on the determination of the molar mass of a gas (be specific: no effect; over- or under- estimate) ? Justify your response.

i. test tube with the gas was partially submerged in the hot water bath.

ii. after the volatile liquid evaporated, you did not immediately condense the gas (since you were too busy talking to your classmate).

2. An unknown mass of zinc reacts with excess hydrochloric acid to produce hydrogen gas and zinc chloride. 75.0 mL of gas was collected over water; the temperature and pressure of the sample is 55.0 ° C and 785 torr, respectively. [10 points]

a. write the balanced chemical equation of this reaction.

b. determine the number of moles of gas produced in the reaction.

c. determine the mass of zinc in the reaction.

 

3. The empirical formula of a (hypothetical ?) gas is B2H3. What is its chemical formula if the gas effuses 5 time slower than hydrogen gas ? [10 points]


4. What is the pressure when

4.0 L Ne at 2.0 atm

2.0 L He at 3.0 atm

2.0 L Ar at 5.0 atm

are mixed and placed in a 12-L container ? The temperature is constant. [10 points]

 

5. old topic: Beer’s law lab: spectrophotometric assay [10 points]

a. what is a calibration curve & its purpose ?

b. a solution of KMnO4 is purple. What color of light – purple or red – would be best in a colorimetric assay for potassium permanganate ? Justify your answer.