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How many grams of a solution that is 12.5% by mass [tex]AgNO_3[/tex] would contain 0.400 mol of [tex]AgNO_3[/tex]?

The back of the book says that the answer is 544 g solution, and I got the answer, but only by randomly multiplying and subtracting the percentage, number of moles, molar mass, and the number 2. All I'd really like for this one is the equation to solve it. The book instructed mass% = [tex]\frac{g solute}{g solution} \times 100[/tex] as the main equation to use for problems like this. Is this right, because I tried converting the equation, but I got a different answer.

Problem 2:

What is the molar mass of a compound if 4.80 g of the compound dissolved in 22.0 g of [tex]H_2O[/tex] gives a solution that freezes at [tex]-2.50^\circ C[/tex]?

Thanks. :)