Three acids of phosphorus have two phosphorus atoms…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Three acids of phosphorus have two phosphorus atoms. Hypophosphoric acid, , has a P-P bond, one P=O and two OH groups on each phosphorus atom. Pyrophosphorous acid, , has a P-O-P bridge, and each phosphorus atom has one P=O, one OH group and one P-H bond. Pyrophosphoric acid, , has a P-O-P bridge and two OH groups with one P=O on each phosphorus atom. Find the oxidation state of phosphorus in each acid. Then find the amount of sodium hydroxide that neutralises of pyrophosphorous acid completely.
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
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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. Use the sum of oxidation states, and then count the P-OH groups.
Step 1. . Six oxygen atoms give and four hydrogen atoms give . The two phosphorus atoms give , so each is . The P-P bond does not change this.
Step 2. . Seven oxygen atoms give and four hydrogen atoms . The phosphorus atoms give , so each is .
Step 3. . Five oxygen atoms give and four hydrogen atoms . Together the phosphorus atoms give , so each is .
Step 4. The base. has two P-OH groups and two P-H bonds, so it is dibasic. of sodium hydroxide.
Answer: , and , and of base.
Check. The hydrogen atoms on phosphorus do not count. If they did, the base would be , which is wrong.
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