steps to solve for new % (m/v) concentration and new molarity ?

Question

a. What is the new % (m/v) concentration when water is added to 25.0 mL of 20.0% (m/v) NaOH to make 300.0 mL of a diluted NaOH solution?

b. What is the new molarity of the solution when water is added to 125.0 mL of 2.50 M NaOH to make 300. mL of a diluted NaOH solution?

Answer 1

In Chemistry, there is an easy formula for finding volume or concentration when doing dilution problems (which this is). It is as follows: V1C1 = V2C2

(a) V1 = initial volume = 25 ml

C1 = initial concentration = 20%

V2 = final volume = 300 ml

C2 = final concentration = ?

(25 ml)(20%) = (300 ml)(x%)

x = 1.67%

(b) V1C1 = V2C2

(125 ml)(2.5 M) = (300 ml)(x M)

x = 1.04 M

Answer 2

% (m/v) is defined by mass of solute in grams per mL of solute times 100 for the percentage. First solve for how much grams of solute you have:

(20g/mL)/100*25mL=5g NaOH

Then to solve for the new % (m/v) you know how much mass there is in the solution. divide by the new total volume (25mL +300mL)and times 100 to get your answer.

b. is the same exact process to solve the problem, but you wont have times 100 anywhere for the percentage and for the mass unit of measurement you have moles i.e. ( 1 M= 1mole/L)