How Do You Calculate Molarity in Diluted Solutions?

[nemzy]

Lets say you pipet into a 250 mL volumetric flask the following:

10 ml of 3e-4 M bromscresol green solution
25 mL of 1.6 M acetic acid (HC2H3O2)
10 mL of .200 M KC1 Solution

and diluted to the 250 mL mark

What is the molarity of acetic acid in this solution?

Do you calculate the number of moles in the 25 mL solution since you know the molarity, then when u get the moles you find molarity over 250 mL?

2nd problem.. Let's say you have a solution with the following:

10 mL of 3e-4 Bromscresol green solution
10 mL of .160 M sodium acetate solution
and diluted to 250 mL mark

What is the mles acetate present?

Do you calculate the # of moles over just 10 mL?

Does the # of moles stay constant even though you add more volume? I think i heard that somewhere but not sure

 

[KLscilevothma]

Do you calculate the number of moles in the 25 mL solution since you know the molarity, then when u get the moles you find molarity over 250 mL?

Yes, this is the way we tackle the problem.

Do you calculate the # of moles over just 10 mL?

] Yes, you need to calculate the number of moles of sodium acetate in 10mL first.

Does the # of moles stay constant even though you add more volume?

Yes. Number of moles tells you the number of molecules present and the number of molecues will not change upon dilution. Let me give you an analogy. If there are 3 balls in a cylinder containing 50mL water, you cannot change the number of balls inside by adding more water into the cylinder.