Question

4. Calculate the mole fraction of phosphoric acid (H3PO4) in a 25.4% (by mass) aqueous solution. (Assume 500 mL of solution.) Please show work

Answer 1

1)

consider 500 ml of solution

assuming the density of solution as 1 g / ml

now

mass = density x volume

so

mass of soltuion = 1 x 500

mass of solution = 500 g

now

we know that

% mass = mass of H3P04 x 100 / mass of solution

so

25.4 = mass of H3P04 x 100 / 500

mass of H3P04 = 127

now

we know that

moles = mass / molar mass

so

moles of H3P04 = 127 / 98 = 1.296

now

mass of water = 500 - 127 = 373

so

moles of water = 373 /18 = 20.722

now

total moles = 1.296 + 20.722 =22.018

now

mole fraction of H3P04 = moles of H3P04 / total moles

so

mole fraction of H3P04 = 1.296 / 22.018

mole fraction of H3P04 = 0.059

so

the mole fraction of H3P04 is 0.059