Question

4. In general chemistry labs, a gas is often collected over water. The total pressure inside a collection bottle is represented by Protal= Pgas + PH2o. The vapor pressure of water at 23.0 °C is 21.07 torr. Hydrogen gas produced from the following reaction. Zn (s) + H2SO4 (aq) → ZnSO4(aq) + Hz (g) a) If 157 mL of wet H2 is collected over water at 23.0°C and a barometric pressure of 738.0 torr, calculate the moles of H2 that are produced. b) Calculate the grams of Zn that have been consumed.

Answer 1

a)

P = 738.0 torr - 21.07 torr

= 716.93 torr

Given:

P = 716.93 torr

= (716.93/760) atm

= 0.9433 atm

V = 157.0 mL

= (157.0/1000) L

= 0.157 L

T = 23.0 oC

= (23.0+273) K

= 296 K

find number of moles using:

P * V = n*R*T

0.9433 atm * 0.157 L = n * 0.08206 atm.L/mol.K * 296 K

n = 6.097*10^-3 mol

Answer: 6.10*10^-3 mol

b)

From reaction,

Mol of Zn reacted = mol of Zn formed

= 6.097*10^-3 mol

Molar mass of Zn = 65.38 g/mol

use:

mass of Zn,

m = number of mol * molar mass

= 6.097*10^-3 mol * 65.38 g/mol

= 0.3986 g

Answer: 0.399 g