Finding Molar mass using the ideal gas law

Question

Mass of Unknown Metal	0.0527g
Volume of H2 Gas Produced	25.5 mL
Barometric Pressure	755 mm Hg
Temperature of Water	298 K

What is the molar mass of the metal used in this experiment? Make sure to attach your work. Use correct number of significant figures.

Answer 1

Molar mass is usually given in units of grams / mole. We are given the grams (0.0527 g), so we can use the ideal gas law (PV = nRT) to find the number of moles.

P = pressure in atm = 755 mm Hg x 1 atm / 760 mm Hg = 0.9934 atm

V = volume in liters = 25.5 ml x 1 L / 1000 ml = 0.0255 L

n = moles = ?

R = gas constant = 0.0821 Latm/Kmol

T = temperature in Kelvin = 298K

Solving for n (moles), we have...

n = PV/RT = (0.9934)(0.0255) / (0.0821)(298)

n = 0.001035 moles (this does not take into account the vapor pressure of water which wasn't provided)

molar mass = g / moles

molar mass = 0.0527 g / 0.001035 moles

molar mass = 50.9 g / mole (3 sig. figs.)