Question

I used 0.35mL of 85% phosphoric acid. Only knowing the volume used and that the density is 1.7, what is the moles of phoshporic acid?

Answer 1

volume = 0.35 mL
density = 1.7 g/mL
So,
mass of solution = density * volume
= 1.7 g/mL * 0.35 mL
= 0.595 g

Now use:
mass % = mass of H3PO4 * 100 / mass of solution
85 = mass of H3PO4 * 100 / 0.595
mass of H3PO4 = 0.5058 g

Molar mass of H3PO4,
MM = 3*MM(H) + 1*MM(P) + 4*MM(O)
= 3*1.008 + 1*30.97 + 4*16.0
= 97.994 g/mol


mass(H3PO4)= 0.5058 g

number of mol of H3PO4,
n = mass of H3PO4/molar mass of H3PO4
=(0.5058 g)/(97.994 g/mol)
= 5.16*10^-3 mol
Answer: 5.16*10^-3 mol