## 6B. 11

Lindsey Chheng 1E
Posts: 110
Joined: Fri Aug 30, 2019 12:16 am

### 6B. 11

A student added solid Na2O to a volumetric flask of volume 200.0 mL, which was then filled with water, resulting in 200.0 mL 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. (a) What is the molar concentration of hydroxide ions in (i) the diluted solution, (ii) the original solution? (b) What mass of Na2O was added to the first flask?

Julie Park 1G
Posts: 100
Joined: Thu Jul 25, 2019 12:15 am

### Re: 6B. 11

Once you find the concentration of the hydroxide ion in the original solution, you'll want to take other given info -- volume of NaOH (0.200L) , ratio of moles of Na2O and NaOH (1mol Na2O and 2mol NaOH according to the balanced reaction equation), and the grams of Na2O per mole (61.98g/mol) -- to use dimensional analysis and finally calculate the mass of Na2O.

Eileen Si 1G
Posts: 120
Joined: Fri Aug 30, 2019 12:17 am

### Re: 6B. 11

To find the mass of Na2O in the first flask, you take the molar concentration of NaOH that you calculated in part aii) and divide by 0.2 L (200 mL) as given in the problem to cancel out the volume and just get moles of NaOH. Then, you need to use stoichiometry to convert moles of NaOH to moles of Na2O by balancing the chemical reaction so Na2O + H2O = 2NaOH. Next, you just need to convert moles of Na2O to grams by using the periodic table and finding the molar mass of Na2O.

Morgan Carrington 2H
Posts: 54
Joined: Wed Nov 14, 2018 12:22 am

### Re: 6B. 11

Lindsey Chheng 1E wrote:A student added solid Na2O to a volumetric flask of volume 200.0 mL, which was then filled with water, resulting in 200.0 mL 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. (a) What is the molar concentration of hydroxide ions in (i) the diluted solution, (ii) the original solution? (b) What mass of Na2O was added to the first flask?