# finding mass-volume percent

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Let's look at an example where we're finding the mass/volume percentage.
The same basic procedure will apply regardless of the percentage, what's going to differ
is whether I'm doing mass over mass, volume over volume, or mass volume.
So here we can look at the example with mass over volume.
And the first thing I do is remind myself what that means.
If I'm trying to find the percent
mass over volume, then I need to know the mass
of my solute over
the volume of the solution.
The thing I need to remember is that the volume of the solution
is equal to the volume
of the solute
plus the volume of the solvent.
So I have to remember to include both of those.
So the first thing I want to look through - look for - in my problem is what do I know?
I've got 15 grams of ethanol.
We're trying to find the mass percentage of ethanol - that tells me that is my solute.
And then it gives me the density, and then I'm given 100 grams of acetone - that is my solvent.
So now what I can do is take the mass of the solute, which I already know.
So percent in v = 15.0 grams of ethanol
and on the bottom I have a little more to do because I've got to find the
volume of the ethanol. And I know the mass is 15.0 grams
and I can use it density 0.789 grams per milliliter
plus the mass of the solvent, 100.0 grams of acetone
and it's density 0.791 grams per milliliter.
Now I'm showing this all in one calculation. I can actually
break apart this and do this separate, and this separate and then combine the volumes
and put them in the equation; either way, whatever works for you.
But this is kind of combining the steps all into one.
So, now what I have is, on the bottom I have 15 divided by .789
and there I get a volume, I'm going to rewrite this as percent m/v equals
15 grams on top - oops forgot my times 100 there -
15 grams on top and so far I have 19 milliliters there on the bottom.
Make that 19.0 milliliters so I have my sig fig (significant figures) correct
and I'm going to add to that my 100 grams divided by .7891 from the acetone
and I get 126 milliliters there.
Now I can add those two numbers together, so 19 + 126
I get 145. And I take 15 divided by 145
times 100 and I get a percentage of 10.3 percent m/v.
And I do need to include that m/v in there so I know what type of percentage it is.
If I were to do the same solution here, but I found mass percent versus
volume/volume percent what I would find is that I would get three very different answers for those.
So I want to make sure I'm always looking at the right type of percentage.