Consider a solution made by mixing 500.0 mL of 0.04182 M Na2HAsO4 with 500.0 mL of 0.03645 M NaOH. Complete the mass balance expressions below for Na and arsenate species in the final solution.
There are 4 arsenate species, but I only got three of them. I am unsure of what the last one could be.
The ones I have are HAsO4^2-, H3AsO4, AsO4^3-. I thought the last one would be H2AsO4^- but it wasn't correct.
Ah, the elusive fourth arsenate species! It's like the missing sock in a pair, always causing confusion. But fear not, my friend, for I've got a clownishly creative solution for you.
The last arsenate species you're missing is none other than... *drumroll*... "H4AsO4"! Yes, you heard it right, we've got ourselves a quad-armed arsenate here. It's just like a regular tricep, but with an extra dose of arsenic flair.
So, go ahead and complete those mass balance expressions for Na and arsenate species in the final solution. And remember, when it comes to chemistry, always expect the unexpected... like a fourth arsenate species itching to join the party!
The fourth arsenate species you are missing is actually H2AsO4-. Here are the complete mass balance expressions for Na and arsenate species in the final solution:
1. Na+: The final concentration of Na+ can be determined by considering the Na+ ions present in Na2HAsO4 and NaOH.
Moles of Na+ from Na2HAsO4 = (500.0 mL) x (0.04182 mol/L) = 20.91 mmol
Moles of Na+ from NaOH = (500.0 mL) x (0.03645 mol/L) = 18.23 mmol
Total moles of Na+ in the final solution = 20.91 mmol + 18.23 mmol = 39.14 mmol
Mass balance expression for Na+:
Moles of Na+ in the final solution = 39.14 mmol
2. HAsO4^2-: This species is formed by the dissociation of Na2HAsO4 in water.
Na2HAsO4 (s) --> 2 Na+ (aq) + HAsO4^2- (aq)
Since Na2HAsO4 dissociates into 2 Na+ ions and 1 HAsO4^2- ion, the concentration of HAsO4^2- in the final solution is equal to the initial concentration of Na2HAsO4.
Mass balance expression for HAsO4^2-:
[HAsO4^2-] = 0.04182 M
3. H3AsO4: This compound is formed from the reaction between Na2HAsO4 and NaOH, where HAsO4^2- reacts with Na+ to form H3AsO4.
HAsO4^2- (aq) + 3 Na+ (aq) + H2O (l) --> 3 H2O (l) + 2 H2AsO4^- (aq) + NaOH (aq)
Since HAsO4^2- reacts with 3 Na+ ions to form 2 H2AsO4^- ions, the concentration of H3AsO4 in the final solution is twice the initial concentration of HAsO4^2-.
Mass balance expression for H3AsO4:
[H3AsO4] = 2 × [HAsO4^2-] = 2 × 0.04182 M = 0.08364 M
4. H2AsO4^-: This species is formed from the reaction between Na2HAsO4 and NaOH, where HAsO4^2- reacts with Na+ to form H2AsO4^-.
HAsO4^2- (aq) + 2 Na+ (aq) + H2O (l) --> 2 H2O (l) + H2AsO4^- (aq) + NaOH (aq)
Since HAsO4^2- reacts with 2 Na+ ions to form 1 H2AsO4^- ion, the concentration of H2AsO4^- in the final solution is half the initial concentration of HAsO4^2-.
Mass balance expression for H2AsO4^-:
[H2AsO4^-] = (1/2) × [HAsO4^2-] = (1/2) × 0.04182 M = 0.02091 M
Hence, the complete mass balance expressions for Na and arsenate species in the final solution are as follows:
1. Na+: Moles of Na+ in the final solution = 39.14 mmol
2. HAsO4^2-: [HAsO4^2-] = 0.04182 M
3. H3AsO4: [H3AsO4] = 0.08364 M
4. H2AsO4^-: [H2AsO4^-] = 0.02091 M
To determine the missing arsenate species, we need to consider the reactions that occur when Na2HAsO4 and NaOH are mixed.
Since Na2HAsO4 is a salt that dissociates into ions in water, it will produce HAsO4^2- ions.
NaOH, on the other hand, dissociates into Na+ and OH- ions in water. However, the OH- ions will react with H+ ions from HAsO4^2- to form water. As a result, the HAsO4^2- species will be converted into another species.
Let's write the balanced chemical equation for this reaction:
Na2HAsO4 + 2 NaOH -> 2 Na+ + HAsO4^2- + 2 H2O
From the balanced equation, we can see that one HAsO4^2- reacts with two OH- ions to form water. This means that the remaining species after the reaction will be the fully deprotonated form of arsenate, which is AsO4^3-.
Therefore, the missing arsenate species is AsO4^3-. The mass balance expressions for Na and the arsenate species in the final solution would be:
Na+: 2 * (0.04182 M) + 2 * (0.03645 M)
HAsO4^2-: 0.04182 M
H3AsO4: 0 M (assumed to be negligible)
AsO4^3-: 0 M
OH-: 2 * (0.03645 M)
H2O: 2 * (0.03645 M)
Note: The expression for H3AsO4 is assumed to be 0 M because Na2HAsO4 is a strong base, and it will almost completely deprotonate in solution.