[trishasales714]

I have the percent weight and the amount used but I don't remember how to go about with the calculations.

[Dan]

Post the actual question please.

What is the definition/formula for percent weight (%wt)?

[trishasales714]

Post the actual question please.

What is the definition/formula for percent weight (%wt)?

Sorry. I do know that the percent weight is (mass of solute/mass of solution) x 100% but I forgot how to do it with solutions. Here's the data: 10% wt. HBr in acetic acid. I'm just trying to figure out the moles for this. I know it could be 0.10 g HBr in 1.0 g of HBr/acetic acid. So would it just be 0.10 g x molar mass of HBr to calculate the moles?

[Dan]

So would it just be 0.10 g x molar mass of HBr to calculate the moles?

No, molar mass is g/mol and you want to convert g to mol

If you do mass × molar mass, the resulting quantity will have the units: g × (g/mol) = g²/mol - but you want mol, so you can see it's not right.

You're not far off, try again.

[trishasales714]

It would be 0.10 g/ molar mass of HBr. But I'm curious if the assumption of 0.10 g is correct? I mean it makes sense with the percent weight if there's 1.00 g of solution but I'm not sure if I can just pick a a random weight for Br (in this case, I said 0.10g).

I forgot to mention that I used 10 ml of 10% wt HBr in acetic acid.

10 ml x density of HBr = #. #/molar mass of HBr = moles.

I'm a bit doubtful about using the molar mass and density of HBr since it is a solution of HBr in acetic acid. Is the above calculation correct or am I supposed to use the density and the molar mass of the combined HBr and acetic acid? I feel like I also haven't taken into account the fact that the 10 ml is not just HBr.

So would it just be 0.10 g x molar mass of HBr to calculate the moles?

No, molar mass is g/mol and you want to convert g to mol

If you do mass × molar mass, the resulting quantity will have the units: g × (g/mol) = g²/mol - but you want mol, so you can see it's not right.

You're not far off, try again.

[Dan]

Yes, mass / MW = moles  (because g/(g/mol) = mol)

OK, so let's nail this down with no ambiguity. Is this your complete question: how do I calculate the moles of HBr in 10 mL of 10%wt HBr(acetic acid) solution?

You need the density of the solution (usually printed on the bottle or listed on the supplier's website, otherwise you will need to measure it), not the density of pure HBr or pure acetic acid. You don't need the MW of acetic acid, just as you don't need the MW of water to do similar calculations for aqueous solutions.

Volume of solution  Mass of solution    Mass of HBr  Moles of HBr