Question

A common way to make hydrogen in the labratory is to place a metal such as zinc in hydrochloric acid. The hydrochloric acid reacts with the metal to produce hydrogen gas, which is then collected over water. Suppose a student carries out this reaction and collects a total of 154.4 mL of gas at a pressure of 742 mmHg and a temperature of 25 degrees Celcius. What mass of hydrogen (in mg) does the student collect?

Answer 1

Subtract out the water vapor pressure as you were given the total pressure of the collected gas including the water vapor.

V=154.4 mL (= .1544 L)
P(Total)= 742 mmHg (=.9763 atm)
P(H2O) @ 25 C= 23.78 mmHg (=.031289 atm)
T= 25 C (=298K)

P(total)= p(H2O)+p(H2) --> p(H2) +.031289=0.9763

So p(H2) = 0.9763 atm - 0.031289 atm = 0.9450 atm H2

n=(PV)/(RT) --> (0.9450 atm * .1544L)/(.08206 * 298K)=
.00597 mol H2

.00597 mol H2 * 2.016g H2 * 1000mg = 12.029 mg H2

This rounds to 12.0 mg