You need a 75% alcohol solution. On hand, you have a 250 mL of a 20% alcohol mixture. How much pure alcohol will you need to add to obtain the desired solution?

You will need
mL_____ of pure alcohol
to obtain
mL______ of the desired 75% solution.

Question

You need a 75% alcohol solution. On hand, you have a 250 mL of a 20% alcohol mixture. How much pure alcohol will you need to add to obtain the desired solution?

You will need
mL_____ of pure alcohol
to obtain
mL______ of the desired 75% solution.

anonymous · Answer

If anyone can explain in steps that would be great, thanks!

kropot72 · Answer

Let the xtra amount of alcohol needed = x (mL)
The amount of alcohol in the original solution = 250 * 0.2 = 50 mL
The final total amount of 75% alcohol solution = (250 + x) mL
The amount of alcohol in the 75% alcohol solution = (50 + x) mL
$$0.75= \frac{50+x}{250+x}$$
Now you need to solve for x.

anonymous · Answer

Thank you!

kropot72 · Answer

You're welcome :) Come back if you need more help on this one.

kropot72 · Answer

0.75(250 + x) = 50 + x
187.5 + 0.75x = 50 + x
0.25x = (187.5 - 50) = 135.5
$$x=\frac{135.5}{0.25}$$

anonymous · Answer

Thank you :)