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Postby JamieVu_2C » Sun Jan 19, 2020 2:29 pm

A student added solid Na2O to a volumetric flask of volume 200.0 mL, which was then filled with water, resulting in 200.0mL of NaOH solution. Then 5.00 mL of the solution was transferred to another volumetric flask and diluted to 500.0 mL. The pH of the diluted solution is 13.25.
(b) What mass of Na2O was added to the first flask?

So for part a I have that [OH-] = .18M for the diluted and original solution. For part b, the equation would be Na2O(s) + H2O(l) --> 2NaOH(aq). How would I know how to calculate the mass of Na2O from the concentration of OH-? Does it matter if I use the diluted or original concentration of OH-?

Jacob Puchalski 1G
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Re: 6B.11B

Postby Jacob Puchalski 1G » Sun Jan 19, 2020 3:59 pm

For part a, since M is moles per liter, the [OH-] of the original solution would be 100 times stronger, since the 5.0 mL solution was diluted to 500 mL. So the original [OH-] would be 18M.

For part b, you use the equation you have to convert the 18M OH- to grams of Na2O.
18 mol/L x .2 L x 1 mol Na2O/2 mol NaOH x 61.98 g Na2O/mol = approximately 110 g Na2O.

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Re: 6B.11B

Postby ASetlur_1G » Sun Jan 19, 2020 10:51 pm

You have to use the original concentration of OH- because they're asking for the initial amount of Na2O. Thus, you want the concentration of OH- before the dilution, which would be 18 M.

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