A sample of 25.00 g of borax, Na₂B₄O₇· 10H₂O, is heated gently until it loses all its water, and…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A sample of of borax, , is heated gently until it loses all its water, and then strongly until it forms a clear glass of sodium metaborate and boron(III) oxide: . The molar masses are for borax, for , for and for . Find the mass of water lost, the mass of anhydrous left, and the masses of and in the glass.
Need a reminder? The key ideaShow
Borax reacts with acid as . Heating gives . For a weak acid, , and a mixture of the acid and its conjugate base obeys . By Hess's law, .
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. Follow the moles through the two stages.
Step 1. Moles of borax. .
Step 2. Anhydrous residue. .
Step 3. Water lost. . As a check, .
Step 4. The glass. : . : .
Answer: of water, of residue, of and of .
Check. , the same as the anhydrous residue, since mass does not change in the second stage.
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