Verified answer

Molar mass of Mg(NO3)2 is 148.31 g/mol

Molar mass H2O = 18.00g/mol

A solution with mol fraction Mg(NO3)2 of 7.4*10^-2 has :

7.4*10^-2 mol Mg(NO3)2 dissolved in 1.00 - ( 7.4*10^-2) = 0.926 mol H2O

Mass of 7.4*10^-2 mol Mg(NO3)2 = (7.4*10^-2) * 148.31 = 10.97g

Mass of 0.926 mol H2O = 0.926*18.00 = 16.67g

Total mass of solution = 10.97+16.67 = 27.64g

% by mass Mg(NO3)2 = 10.97/27.64 * 100 = 39.7%m/m

Pretty sure you just convert to a percent by *100.

so 7.42%. Tell me if this is wrong though