Question

I may need this broken down step by step to understand this problem thanks. A 20.0% by mass solution of phosphoric acid (H3PO4) in water has a density of 1.114 g/ml at 20℃. What is the molarity of the solution?

Answer 1

I could just give you a formula that will do this; I memorized it years ago but here is a question by question approach.
What is the definition of molarity.
M = mols/L. So that's what we need to determine.
How much does a L weigh? That's
1.114 g/mL x 1000 mL = 1114 grams. Simple enough, eh?
How much of that 1114 g is H3PO4. It says it is 20% by mass, therefore, 1114 g x 0.20 = 222.8 g.
How many mols are there in 222.8 g H3PO4? That's mools = grams/molar mass = 222.8/98 = 2.273 mols.
The definition of M is what? M = mols/L. And you have 2.273 mol/L; that must be the molarity.

The formula is
density g/mL x 1000 mL x mass% x (1/molar mass) = M
1.114 x 1000 x 0.20 x (1/98) = 2.273 M.
Have a good day.

Answer 2

a liter of solution has a mass of 1.114 kg

20.0% of that is H₃PO₄ ... 222.8 g

the molar mass of H₃PO₄ is ... 97.99 g

so the moles/L is ... 222.8 / 97.99

the 20.0 means 3 sig fig