# Question #278e0

##### 2 Answers

You would need to add

So, you know that your final solution is

Now, this amount of ethanol must come from two sources: the absolute ethanol - I'll label this

**(1)**, and

**(2)**

Now, you can express the volume of

Plug this into equation **(2)** and you'll get

So,

You have

Solving this will get you

This value represents the amount of ethanol you must get from the

If you round this value to one sig fig, the answer will be

The student should use 790 µL of 32 % ethanol.

Let

Then we can re-write the problem as

Cancelling units, we get

**(1)**

We also know that the total volume must be 900 µL.

**(2)**

We have two equations in two unknowns.

Solve (2) for

**(3)**

Substitute (3) into (1).

So you would use 790 µL of 32 % ethanol.

Note: The answer can have only 2 significant figures, because that is all you gave for the

32 % ethanol. If you need more precision, you will have to recalculate.