Answer:
Pure water =74.1 mL
Step-by-step explanation:
You need 420 mL of a 70% alcohol solution.
420 mL of solution and 70% of it should be alcohol.
So,
420 * 70/100 mL
= 420 * 0.7
= 294 mL of alcohol and the rest should be water.
On hand, you have a 85% alcohol mixture.
The alcohol can only come from the 85% mixture. So, how much of 85% alcohol do we need to get 294 mL of alcohol:
Let
y = alcohol solution needed
85% of y = 294
85/100 * y = 294
y = 294 ÷ 85/100
y = 294 × 100/85
y = 29,400/85
y = 345.88235294117
Approximately, 345.9mL
The rest of 420 mL solution has to be pure water
Pure water = 420 mL - 345.9mL
= 74.1 mL
Pure water =74.1 mL
