What will be the molarity of the solution?

Question

What will be the molarity of the solution made by mixing 250 mL of 0.75 M H2SO4 with 250 mL of 0.70 M H2SO4?

How do you set this problem up?

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Answer 1

Molarity is moles of solute per liter of solution. The volume of the final solution will be (250 + 250)ml = 500 ml, so now all we have to do is find the number of moles of the solute. You have two different H2SO4 solutions of known volume and concentration so you can easily find the moles of H2SO4 (solute) in each solution and add them together to get the total moles of solute in the final solution.

For the first H2SO4 solution:

(250 ml)(L/1000 ml)(0.75 mol/L) = 0.1875 moles of H2SO4

For the second H2SO4 solution:

(250 ml)(L/1000 ml)(0.70 mol/L) = 0.175 mole sof H2SO4

total moles of solute  = 0.1875mol + 0.175 mol = 0.3625

molarity of final solution:

0.3625 moles/0.5 L = 0.725 M

Recall that (500ml)(L/1000 mL) = 0.5 L

Answer 2

Step 1.

work out the number of moles in each part of the solution.

n(moles) solution 1 = Conc x Vol (L)

              = 0.75 x .25

              = 0.1875moles

n(moles) solution 2 = 0.7 x 0.25

             = 0.175

step 2.

add the number of moles together.

0.1875 + 0.175 = 0.3625 moles

step 3.

final concentration = moles / vol (litres)

               = 0.3625 / 0.500       <== two lots of 250mls gives 0.5L

               = 0.725 M H2SO4

follow this method and it will work for all of this type of question. for questions with the same initial volume of solutions you can add the two concentrations together, divide by 2 and youll get the same answer. ONLY if the volumes are the same.

eg 0.7 + 0.75 = 1.45

divide by two and you get 0.725 M the same answer as above.