G.25 Dilution

[ngarcia]

Hi all,

I am currently working on problem G.25. If you dilute 10. mL of a solution and double the volume 90 times, would you simply multiply 10. mL by 90 and cancel out the mL/L with the molar concentration of the substance to find the number of molecules (using Avogadro's number)?

Thank you!

[Nicholas Chin 1G]

So if the wording is "double the volume 90 times," that means you have to multiply 10 mL by 2 90 times. An easy way to do this is just 10 mL * 2^90, which effectively "doubles the volume 90 times" since you multiply by 2 90 times. Now you have a volume initial (10 mL), a molarity initial (0.10 mol/L), a volume final (10 mL * 2^90), and you want to find the molarity of the final solution. So using the formula M1 V1 = M2 V2, plugging in your knowns, you get M2, the molarity of the new solution. After this, you can find the moles of X in 10 mL of the new solution since you found M2. And finally, you can use Avogadro's number with the amount of moles you found to find the molecules of X in the solution.

Hope this helped! Let me know if you want me to clarify anything.

[Deepika Reddy 1A]

Hi,
When you dilute a solution, the number of moles will not change, so the molecules will be the same before and after you dilute because the moles are the same.

[romina_4C]

Starting from the beginning you can use the MiVi = MfVf equation. Thus, you have the original molarity of the solution times 0.01 L (convert milliliters to liters) = the new/unknown concentration (can give it variable x) times 0.09 L (90 ml converted to liters). Thus you find the molarity by finding the variable x, since molarity = moles/liter, you would multiple the new molarity by 0.09 L to get the moles and then use avogadro's number.

[VPatankar_2L]

I don't think you can multiply 10 mL by 90 to get the answer. If you're doubling the volume each time, you would be going from 10 mL to 20 mL to 40 mL to 80 mL to 160 mL and so on, and the concentration would continue to be halved. To find the final concentration, you should multiply 0.1 mol/L by (0.5)^90.
After you get this final concentration, multiply by the volume (10 mL) to get the number of moles.

Hope this helps.