How to calculate Percent by Mass.

For this particular set of questions, you have to know what  solute and solvent are. Solute is the mixture or the thing that mixes. Solvent is the thing the solute mixed in. So let's say  I had sugar and water. Sugar would be the solute because it mixes it with the water (solvent).

Now that we have that cleared. we have to memorize a little thingy.

It is   mass of solute/mass of solution times 100.

Okay.
So they gave us a problem that looks like this:
What is the percent by mass of  CO2 in a solution containing 13 g  CO2 dissolved in 400 mL of water.

Look at the problem again. If you read this carefully you can see that dissolved is in bold. Now what dissolved in the water? The 13 grams of CO2!!!

That means the solute is 13 and the solvent is water.
Now we add the solute and the solvent.
13+ 400= 413g.

Now we know the mass of solution.
Look at the eqaution above.
mass of solute/mass of solution times 100.

Just plug in the numbers.
13/413 times 100= 3.14%

 So the answer for this equation is 3.14 %.

Any other questions, please comment!