PV=nRT equation, I think?

Question

Calcium carbide (Ca₂C) reacts violently with water to make calcium hydroxide and acetylene gas (C₂H₂) as shown below:

Ca₂C (s) + H₂O (l) → Ca(OH)₂(aq) + C₂H₂(g)

 

If 14.8 L of acetylene gas is collected over water at a temperature of 52 ^oC and a pressure of 2,385 mmHg, and the vapor pressure of water at 52 ^oC is 108 mmHg, how many g of Ca₂C are required? Give your answer to the nearest tenth of a g. 

Answer 1

When gas is collected over water you need to subtract the vapor pressure of water from the total pressure to obtain the pressure of the gas. Then use PV = nRT.

P = pressure = 2385 - 108 = 2277 mm x 1atm/760 mm = 2.99 atm

V = vol = 14.8 L

n = moles = ?

R = constant = 0.0821 Latm/Kmol

T = temp in Kelvin = 52+273=325K

solve for n…

n= 1.66 moles acetylene

1.66 mol C2H4 x 1 mol Ca2C/mol C2H4 = 1.66 mols Ca2C x 64.1 g/mol = 106.3 g