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.

 

[ravenprp]

To calculate the molarity of acetic acid in the first solution, you would first need to calculate the moles of acetic acid present. This can be done by multiplying the volume (25 mL) by the molarity (1.6 M), which gives you 0.04 moles of acetic acid. Then, to find the molarity in the diluted solution, you would divide the moles by the total volume (250 mL), giving you a molarity of 0.00016 M acetic acid.

For the second problem, to find the moles of acetate present, you would first need to calculate the moles of sodium acetate by multiplying the volume (10 mL) by the molarity (0.160 M), which gives you 0.0016 moles of sodium acetate. Since sodium acetate dissociates into one acetate ion per molecule, you would have 0.0016 moles of acetate ions present. This would be the same whether you calculate it based on the 10 mL or the total volume of 250 mL, as the number of moles of acetate ions would not change with dilution.

The number of moles of a substance does not change when you add more volume to a solution, as long as the amount of the substance remains constant. This is because moles are a measure of the amount of a substance, regardless of the volume it occupies.