Question

If `"250 mL"` of water is added to `"45.0 mL"` of a `"4.2 M KOH"` solution, what will be the final concentration?

Answer 1

The final concentration will be `"64 M KOH"`, or `"64 mol/L KOH"`.

Explanation:

Molarity, `"M"`, is a measure of concentration of a solution, and represents `"mol solute"/"1L solution"`. Milliliters need to be converted to liters. The following is the equation needed to answer this question:

`C_1V_1=C_2V_2`

where concentration is `C` and volume is `V`.

Given/Known Information
`C_1="4.2 M KOH"="4.2 mol KOH"/"L"`
`V_1="45.0 mL"xx"1 L"/"1000 mL"="0.0450 L"`
`V_2="250 mL+45.0 mL"="295 mL"xx"1 L"/"1000 mL"="0.295 L"`

Unknown: `C_2`

Solution
Rearrange the equation to isolate `C_2`. Insert the known values and solve.

`C_2=(C_1V_1)/V_2`

`C_2=(4.2"mol KOH"/"L"xx0.0450color(red)cancel(color(black)("L")))/(0.295color(red)cancel(color(black)("L")))="64 mol KOH"/"L"="64 M KOH"` rounded to two significant figures