A solution has 0.0100 mol each of hypophosphorous acid H₃PO₂ (one OH group), phosphorous acid H₃PO₃…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A solution has each of hypophosphorous acid (one OH group), phosphorous acid (two OH groups) and hypophosphoric acid (four OH groups). It is titrated to the end point, where every OH hydrogen has been replaced, with sodium hydroxide of concentration . A second sample of of pure is treated with an excess of silver nitrate solution. The reaction is . Take the molar mass of silver as . Find the volume of sodium hydroxide for the mixture, the mass of silver, and the amount of sodium hydroxide that neutralises everything acidic in the second sample afterwards ( is tribasic).
Need a reminder? The key ideaShow
The basicity of an oxoacid is the number of X-OH groups. Hydrogen atoms bonded to the central atom are not acidic. The oxidation state comes from oxygen at (at in an O-O link) and hydrogen at , shared among the central atoms. Pauling's rule gives the first pKa as about , for non-hydroxyl oxygen atoms, and each next hydrogen is about 5 units weaker.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Count replaceable hydrogen, then use the equation for silver and for the products.
Step 1. The mixture. of replaceable hydrogen, so of base.
Step 2. The volume. .
Step 3. Silver. Each gives 4 Ag, so . The mass is .
Step 4. After the reaction. The sample now holds of , which is tribasic, so of base. It also holds of free .
Step 5. The base. .
Answer: , and .
Check. Before the silver reaction the sample needs only of base. The reaction makes the solution much more acidic.
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