Question

How would I calculate the mass of 20ml of a 3% hydrogen peroxide from a 30% H2O2 solution?

Answer 1

Is that 3% w/w? w/v? Do you have a density for the 3% solution? I'm still trying to figure out what the 30% solution has to do with your question.

Answer 2

The question says calculate the mass or volume required to make the following solution.
20mL of a 3% hydrogen peroxide solution from a 30% H2O2 solution.

Answer 3

OK. I assume that's 3% w/w and 30% w/w. Then 20 g x 3% = g x 30%
Solve for g = 2g of the 30% and make up to 20 mL .

Another way.
The solution is to be diluted fromj 30$ to 3% or a factor of 10. That'a dilution of 1part H2O2 + 9 parts H2O.
1 part is 20/10 = 2 so 2 mL 30% H2O2 + 9*2 = 18 mL H2O to make 20 mL.