Question

I have .8ml of 35% by weight hydrogen peroxide. The hydrogen peroxide density is 1.14 g/ml. how many moles of hydrogen peroxide are there?

Answer 1

You want to calculate the molarity of the H2O2.
35% w/w means 35 g H2O2/100 g solution.
Since mass = volume x density, then volume = mass/density = 100g/1.14 = about 87.7 mL.
mols in 35 g = 35/about 34) = about 1 mol
M = mols/L = about 1/0.0877 = ?
Then mols in 0.8mL = M x L.
Don't forget to convert 0.8 mL to L.

Answer 2

how many moles are in 4.6ml of 48% HBR? The 48% is by weight. I am not given the density this time.

Answer 3

I don't believe it can be done without the density. I think 48% w/v can be done without density but not w/w.

Answer 4

Thank you.
So it is w/v vs w/w that makes the difference

Answer 5

In this case yes.
If 48% w/v, that means 48g HBr/100 mL solution so 4.6 mL will have
48 g HBr x (4.6 mL/100 mL) = ? g and that gives grams in the volume you want which can be convert to mols by mol = g/molar mass. The problem with w/w is that you have no way of knowing how much the 4.6 mL weighs. With density you have a way of converting volume to weight. You have volume in w/v solutions and no conversion is necessary.