An evaluation of R was performed, following the procedure described in this modu

Question

An evaluation of R was performed, following the procedure described in this module. The barometric pressure was 736 torr, the temperature was 295k and volume of hydrogen gas collected was 35.6 mL. The calculated value of R was 82.1 mL atm K-1 mol-1.

1) How many grams of Magnesium metal were used?

2) If the vapor pressure of water had not been taken into account, what would have been the calculated value of R?

3) If the springe volume had been incorrectly read, giving a calculated system volume of 25.6 mL, what would have been the percent error in the calculated value of R?

Explanation / Answer

1) PV=nRT
First you find moles by using the given information above.
To find the correct pressure you divide 736/760 to convert to atm. = .9684 atm
n=PV/RT (.9684atm) (35.6 mL) / (82.1ml atm/k mol) (295 K) = .0014234 mol
Then you find the molar mass of magnesium metal. Which is 24.31 g/mol
To find how many grams of magnesium metal were used you multiply the molar mass of magnesium metal by the number of moles you calculated.
So... .0014234 x 24.31 g/mol = .035g

2) PV =nRT

P=1-0.9684

P=0.0316

therefore,

0.0316*35.6 = 0.0014234 R 295

R = 2.679

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