A 0.250-g sample of a magnesium-aluminum alloy dissolves completely in an excess

Question

A 0.250-g sample of a magnesium-aluminum alloy dissolves completely in an excess of  HCl(aq). When the liberated H2(g) is collected over water at 29 C and 752 torr, the volume is found to be 325 mL . The vapor pressure of water at 29 C is 30.0 torr.

Part C

Taking into account the vapor pressure of water, how many moles of hydrogen gas, n, are present in 325 mL at 752 torr and 29 C?

The value of the gas constant R is 0.08206 Latm/(molK). You may also find the conversion factors1 atm=760 torr and TK=TC+273 useful.

Express the number of moles to three significant figures.

Explanation / Answer

Part C : using, moles (n) = PV/RT

with,

P = 752 - 30 = 722 torr = 0.95 atm

V = 325 ml = 0.325 L

R = gas constant

T = 29 + 273 = 302 K

we get,

moles of H2 = 0.95 x 0.325/0.08205 x 302 = 0.0085 moles

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