[leah106]

We just did a lab called Standard Heat of Formation, where we used a calorimeter to find the temperature when HCl reacted with Mg. I used 80.1 mL of 3.0 molar HCl, and 0.6000g of Mg.

First thing, I'm asked to calculate the mass of 3.0 molar HCl used.
My thought was that since molarity = mols/volume, that I would plug in the molarity and volume to calculate the number of mols, and then convert to grams. (I got 8.76 g)

Then I was looking through my lab manual, and found a note that say's specifically, "Use the density of a dilute aqueous solution (1.01 g/ml) when calculating the mass of the solution for which you know only the volume." So if that's the case, 1.01 = g/ 80.1 mL, and that means grams would equal 80.9 g.

What is the right way to approach this? Everything was making perfect sense until I read that note. Now I'm afraid all of my calculations are off. :/ Can anyone guide me in the right direction? Why would one of these methods be right, while the other isn't?

[UG]

The second way of using the density is correct. The first method you used only takes into account the mass of HCl alone, without considering the mass of water it is in.

[leah106]

Thank you so much!