CH 101/Exercises 3
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Combined problems
In earlier exercises we have
- balanced equations
- computed molar masses
- converted grams to moles
- moles to numbers of particles
Often we need these skills in combination to solve a problem. Also the wording of the problem is not always a direct clue as to what calculation we are really supposed to do. We often have to 'dissect' the language to figure out what it is we are supposed to do.
Some combined problems
Exercise 1
- To a solution of a salt of a lanthanide metal M 1 mmole Br is added in the form of bromide. This is a small amount compared to the metal in solution. A precipitate is formed and weighed after careful separation and drying. Its mass is 132.3mg. It can be assumed that
- the precipitate has the chemical formula is MBr3
- that all bromine added precipitated
- What was the element in solution?
Exercise 2
- An aliquot of naphthalene C10H8 is burnt in a bomb pressurized to 25 Atm with pure oxygen. The water formed is collected and weighed. Its weight is 1.324 g. What was the weight of the aliquot of naphthalene, assuming the combustion was complete?
Exercise 3
Consider the reaction
- CaC2 + H2O ==> Ca(OH)2 + C2H2
- All the ethyn C2H2 gas is collected and burnt completely with pure oxygen, in which reaction 4.32 g of water are formed.
- How much calcium carbide (by weight) was used assuming that an excess of water was used to react with it?
Exercise 4
Consider the reaction
- CaCO3(s) ==> CaO(s) + CO2(g)
- The pressure is 1 Atm and temperature 25oC.
- What is the volume of carbon dioxide gas evolved when 16kg of calcium oxide is formed?
Exercise 5
A flat piece of titanium with a surface area of 1cm2 is oxidized in oxygen. A 350nm thin layer is formed of anatase TiO2. The density of anatase is 4.3 [g/cm3].
How many titanium atoms does the thin layer contain?
Exercise 6
The compound NH4V3O8 is prepared in three steps:
- N2 + H2 ==> NH3
- NH3 + V2O5 + H2O ==> NH4VO3
- NH4VO3 + HCl ==> NH4V3O8 + NH4Cl + H2O
- How many grams of N2 are needed to prduce 610 g of NH4V3O8, assuming all other reactants are present in abundance
- How much water is consumed/produced in the (total) synthesis.