A piece of aluminium foil of mass 5.40 g is dropped into an excess of sodium hydroxide solution…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A piece of aluminium foil of mass is dropped into an excess of sodium hydroxide solution. The foil dissolves completely and hydrogen forms. Take the molar mass of aluminium as , of sodium hydroxide as and of as . The molar volume of a gas at and is . Find the volume of hydrogen, the mass of sodium tetrahydroxidoaluminate formed and the smallest mass of sodium hydroxide that is needed.
Need a reminder? The key ideaShow
To make a gaseous ion, the three electrons leave one after the other. The energy is the sum . A weighted mean of isotope masses is . Aluminium dissolves in alkali: , so 2 mol of Al give 3 mol of H.
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.
Given. .
Step 1. Moles of aluminium. .
Step 2. Hydrogen. The equation gives mol of H for mol of Al, so . The volume is .
Step 3. The aluminate. Its moles equal the moles of aluminium. The mass is .
Step 4. Sodium hydroxide. Also , so the mass is .
Answer: , and .
Watch out. Students often forget the factor of in step 2 and write the same moles of hydrogen as of aluminium.
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