Question

The gram formula weight of phosphoric acid is 98.00 g/mol, and the density of 85.0% phosphoric acid is 1.685 g/mL. Could you describe how to prepare 500 mL of a 1.0 M solution of phosphoric acid from 85.0% phosphoric acid?

Answer 1

You would add 34 mL of 85 % phosphoric acid to enough distilled water to make a total of 500 mL.

Explanation:

You must make the following conversions:

`"Volume of dilute acid" → "moles of H"_3"PO"_4 → "mass of H"_3"PO"_4 → "mass of conc. acid" → "volume of conc. acid"`

Here we go:

`0.500 color(red)(cancel(color(black)("L dil. acid"))) × (1.0color(red)(cancel(color(black)("mol H"_3"PO"_4))))/(1color(red)(cancel(color(black)("L dil. acid")))) × (98.00color(red)(cancel(color(black)("g H"_3"PO"_4))))/(1color(red)(cancel(color(black)("mol H"_3"PO"_4)))) × (100color(red)(cancel(color(black)("g conc. acid"))))/(85color(red)(cancel(color(black)("g H"_3"PO"_4)))) × (1"mL conc. acid")/(1.685color(red)(cancel(color(black)("g conc. acid")))) = "34 mL conc. acid"`

So, you would add 34 mL of 85 % phosphoric acid to enough distilled water to make a total of 500 mL.

Note: The answer can have only two significant figures, because that is all you gave for the molarity of the dilute acid and the percent concentration of the concentrated acid. If you need more precision, you will have to recalculate.