[ana2345]

I got this problem on a test:
You have an HCl solution that's 1.115 g/mL and is 30% HCl by weight. What is the molarity?

I assumed the density of pure water was 1 g/mL and got 0.115 g of HCl per mL of solution and then calculated the number of moles of HCl per L solution from there, etc. Apparently we were supposed to do 30% * 1.115 g/mL = 0.335 g of HCl per mL, which would have given an answer about 3 times greater than the answer I got.

My question is: shouldn't both of these methods yield the same answer? What was wrong with my method?

[Borek]

Density of 10% solution of ammonia is 09591 g/mL - using your approach, mass of ammonia is negative.

Density of acetic acid solutions first goes up, then goes down, but after reaching maximum at about 80% goes back down. Does it mean 90% solution has less acetic acid than 80% solution?

Also note that your calculations are inconsistent with data - if solution is 30% w/w and 1mL weights 1.115g (btw - it is an incorrect value, it weights about 1.15g), it must contain about 0.3*1.115g - and 0.115g is much less.

[ana2345]

Thanks. That's pretty cool about the acetic acid.

I guess my error was in thinking that 1 mL of solution has 1 mL of water in it. Does this mean that without extra information (assuming you know what the solute is), you can't calculate the molarity of a solution just from the density?

[Borek]

If density is all you have, you need density tables to find concentration.