Task. Calculate the mass of aluminium sulfide if the reaction of sulfuric acid entered the 2.7 g of aluminium.
Recorded a short condition
m(Al) =2, 7 g
m(Al2 (SO4) 3)-?
Before to solve problems in chemistry, a chemical equation. In the interaction of metals with dilute acid forming salt and released gaseous substance is hydrogen. We arrange the coefficients.
2Al + 3H2SO4 = Al2 (SO4) 3 + 3H2
When you always need to pay attention only to substances for which known and you need to find, options. All the rest are not taken into account. In this case it will be: Al and Al2 (SO4) 3
Find the relative molecular mass of these substances in the table of D. I. Mendeleev
Mr(Al2 (SO4) 3) =27•2(32•3+16•4•3) =342
Translate these values into molar mass (M) multiplied by 1 g/mol
M(Al2 (SO4) 3) =342g/mol
Recorded a basic formula, which connects the amount of substance (n), mass (m) and molar mass (M).
Carry out calculations according to the formula
n(Al) =2.7 g/27G/mol=0.1 mol
Make up two ratio. The first relation based on equation based on the odds faced by the formulas of the substances whose parameters are given or need to find.
First ratio: 2 mol Al you have 1 mole of Al2 (SO4) 3
The second ratio of 0.1 mol Al we have X mole Al2 (SO4) 3
(compiled on the basis of the obtained results)
Solve the proportion, given that X is the amount of substance
Al2 (SO4) 3 and has the unit mol
n(Al2 (SO4) 3)=0.1 mol(Al)•1 mol(Al2 (SO4) 3):2моль Al=0.05 mol
Now there is the amount of substance and molar mass of Al2(SO4)3, consequently, you can find lots of which derive from the basic formula
m(Al2 (SO4) 3)=0.05 mol•342g/mol=17,1 g
Answer: m(Al2 (SO4) 3)=17,1 g
At first glance, it seems that to solve chemistry problems is very difficult, but it is not. And to verify the degree of assimilation, first try to solve the same problem, but only yourself. Then substitute other values using the same equation. And the last step is the solution of the new equation. And if you were able to handle what – you're to be congratulated!
Wonderful helper during solving tasks is a manual, time-tested "problems in chemistry for entering Universities" G. P. Khomchenko. And don't be afraid to use it – it proposes the solution of problems from the very beginning!