When an 18.4g sample of a monocarboxylic acid is reacted with magnesium metal, 4.48 liters of hydrogen are produced at STP. What is the formula odf acid?

1 Answer
anor277
May 23, 2018

Well the molar volume of a gas at #"STP"# is #22.4*L#...

Explanation:

And we address the stoichiometric equation...

#2RCO_2H(aq) +Mg(s) rarr {RCO_2^(-)}_2Mg^(2+)+H_2(g)uarr#

#"Moles of dihydrogen"=(4.48*L)/(22.4*L*mol^-1)=0.200*mol#...and so by the stoichiometry there were TWO equiv of the carboxylic acid...

And so we can address the molar mass of the starting acid...

#(18.4*g)/(0.400*mol)=46.0*g*mol^-1#...

And this molecular mass is consistent with that of #HC(=O)OH#, i.e. #"formic acid.."#

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