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
Now, you can express the volume of
Plug this into equation (2) and you'll get
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.
Then we can re-write the problem as
Cancelling units, we get
We also know that the total volume must be 900 µL.
We have two equations in two unknowns.
Solve (2) for
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.