Question

What are the mole fractions of H3PO4 and water in a solution of 14.5 g of H3PO4 in 125 g of water?

(a) Outline the steps necessary to answer the question.

(b) Answer the question.

Answer 1

(a)

Step 1 : Calculate molar mass of H3PO4 and water

Step 2 : calculate moles of water and H3PO4 using mole formula , "Mole = mass in grams / molar mass"

Step 3 : Use mole fraction formula which is "Mole fraction of a substance = moles of that substance / Total moles"

(b)

Let's find molar masses of the given substances

Molar mass of H3PO4 is 3H + P + 4O = 3 * 1.01 + 30.97 + 4 * 16 = 98

Molar mass of water, H2O is 2H + O = 2 * 1.01 + 16 = 18.02

Mole = mass in grams / molar mass

Moles of H3PO4 = 14.5 / 98 = 0.148 mol

Moles of water = 125 / 18 = 6.94 mol

Mole fraction of H3PO4 = moles of H3PO4 / ( moles of H3PO4 + moles of water)

Mole fraction of H3PO4 = 0.148 / ( 0.148 + 6.94) = 0.0209

Mole fraction of water = moles of water / ( moles of H3PO4 + moles of water)

Mole fraction of water = 6.94 / ( 0.148 + 6.94) = 0.979

Mole fraction of H3PO4 is 0.0209

Mole fraction of water is 0.979