THE ELECTRONIC TEXTBOOK IN CHEMISTRY. 15.RESHENIE ZADACH

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15. PROBLEM SOLVING

Determination of relative molecular mass

What relative molecular mass _{of Mr} of phenol
_C6H5OH?

Decision:
Mr=6Ar(C) +5Ar (H)+Ar(O) of +Ar (H) =6*12+5*1+16+1=94

Otvet:94

Determination of mass of atom in grams

What is weight equal in grams of 1 ^{atom 131I} to?

Decision:

1 mol of any substance contains ^6*1023 particles.
Let's find the molar mass of atoms I M (I) of =131 g/mol;
Let's determine the mass of one atom ^{of iodine 131/6*1023=2,18*10-22g}.

^{Otvet:2,18*10-22g}.

Definition of number of molecules in portion of substance

How many water molecules contain in 3 g of substance?

Decision:

Let's find the molar mass _{of water M (H2O)} =1*2+16=18g/mol
Let's find amount of substance ν=m/M=3/18=1/6 mol;
Let's find number of molecules on ^{formula N=ν*Na=1/6*6*1023=1023}

^Otvet:1023 molecules

Determination of amount of the substance corresponding to certain weight.

To what amount of substance there corresponds the mass _{of H2O water} of 9 g?

Decision:

Let's find the molar mass _{of M(H2O) water} =1*2+16=18g/mol
Knowing the molar mass of substance and mass of substance we will find amount of substance on формуле:ν=m/M; ν=9г/18г/моль =0,5 mol.

Answer: 0,5 mol

Calculation of mass of mother substance or reaction product on the known mass of one of substances.

How many of water is formed at complete combustion of 12 g of hydrogen?

Decision:

Let's write down the equation of reaction of burning of hydrogen and over substances we will specify data from statement of the problem:

1. Let's find molar masses of hydrogen and water
M _(H2) =1*2=2g/mol, _M(H2O) =1*2+16=18g/mol

2. Let's find amount of substance of hydrogen, the corresponding 12 g of hydrogen.
ν=m/M; ν=12г/2г/моль =6 mol.

3. From the equation of reaction it is visible that 2 mol of water are formed of 2 mol of hydrogen (we determine by the coefficients facing hydrogen and water). If have burned 6 mol of hydrogen, then also 6 mol of water are formed. We find the mass of water on formula: m=ν*M=6 mol*18 g/mol of =108 g.

Answer: _m(H2O) of =108 g.

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