# Question #e042c

##### 1 Answer

Here's what I got.

#### Explanation:

Start by picking a sample of this

The solution is said to be

Use the **density** of the solution to find its *mass*.

#100 color(red)(cancel(color(black)("mL solution"))) * "0.9 g solution"/(1color(red)(cancel(color(black)("mL solution")))) = "90 g solution"#

Next, usee the **density** of ethanol to find the mass of the solute.

#20 color(red)(cancel(color(black)("mL ethanol"))) * "0.75 g ethanol"/(1color(red)(cancel(color(black)("mL ethanol")))) = "15 g ethanol"#

Now, in order to find the solution's **percent concentration by mass**,

Since you already know how much ethanol you have in

#100 color(red)(cancel(color(black)("g solution"))) * "15 g ethanol"/(90color(red)(cancel(color(black)("g solution")))) = "16.667 g ethanol"#

This means that the percent concentration by mass is equal to

#color(darkgreen)(ul(color(black)("% m/m = 17% ethanol")))#

I'll leave the answer rounded to two **sig figs**, but keep in mind that you *should* round it to one significant figure, the number of sig figs you have for the percent concentration by volume.