6B. 11

[Alejandro_Franco_3B]

How do I approach this question? Do I first have to set up the balanced chemical reaction? Is the molar concentration of hydroxide ions in NaOH the same as the molar concentration NaOH in the solution since it is a strong base?

[Avantika MOhan_2B]

So for the diluted solution, you are given the pH. We can use 10^-pH = the concentration of Hydronium ions and use it to calculate hydroxide concentrations. Now that you have the concentration in the dilute solution, you reverse engineer it to find the concentration of hydroxide ions in the undiluted solution (using c1v1 = c2v2). Once you have the concentration of the original solution you can find the moles using c = n/v and multiply that by molar mass to find the grams of Na2O. Don't forget to write out the reaction to check for the molar ratio when doing the last step.

[Miah Chao 2I]

[PART A]
First, we are given the pH of the diluted solution. To find the concentration of hydroxide ions in the diluted solution (i), we can first use the formula pH+pOH=14 to find the pOH, and then use the formula [OH-]=10^-pOH.

Next, we are asked to find [OH-] of the original solution. We can use the dilution formula M1V1=M2V2, in which M1 is the initial concentration, V1 is the initial volume, M2 is the concentration after dilution, and V2 is the total final volume. M2 is equal to the [H3O+] that we found in part i. V2 is equal to the final volume after dilution which was 500.0 mL. V1, or the initial volume that was diluted is 5.00mL (as this is how much was transferred). By plugging these values in, we can solve for M1, which will represent [OH-] in the initial solution.

[PART B]
This part is a stoichiometry problem. To answer your question, yes, [OH-] in NaOH is the same as [NaOH] because NaOH is a strong base and will completely dissociate into Na+ and OH-.

First, we can start off with the volume of the first flask, which was 200.0 mL, and convert it into liters. Then, we can multiply the volume by [OH-]=[NaOH] which was found in part a ii, allowing us to convert it into moles of NaOH. Note that for every 2 NaOH, there is 1 Na2O, so include this into your conversion while converting to Na2O. From then, multiply the moles by the total molar mass of Na2O and you will get the solution.