Composition stochiometry

Solution:

During the combustion of magnesium, 5–3 = 2 g of oxygen was consumed. The composition of magnesium oxide expressed as the mass ratio of elements is:

m(Mg): m(O) = 3∶2

Solution:

From the percentage composition of the compound, it logically follows that 100 g of the compound contains 32.43 g sodium, 22.55 g sulfur, and 45.02 g oxygen. To determine the ratios of the elements in the compound, we need to know the molar ratios, which are calculated using the molar mass (values taken from the periodic table of elements):

The ratio of molar amounts of the elements in the compound is:

 The obtained ratios are adjusted to whole numbers by dividing by the smallest value:

Thus, one mole of sulfur corresponds to 4 moles of oxygen and two moles of sodium. The empirical formula is Na₂SO₄, sodium sulfate.

Solution:

As in the second example, we first determine the molar ratios of the elements in the compound.

The smallest molar ratio is obtained by dividing by the value for hydrogen:

According to this ratio, the stoichiometric formula is HCO₂. The relative molecular mass of such a compound would be 45.017, exactly half of the given value. To obtain the given value of 90.034, the elements must be doubled in the molecule. The molecular formula is therefore C₂H₂O₄.

Solution:

From the known mass fractions of carbon and hydrogen, we can calculate the mass fraction of oxygen:

w(O) = 100 – (39.85 + 6.75) = 53.40 %

We proceed as in the previous example by calculating the molar amounts

Slight deviations from whole numbers arise from inaccuracies in determining the carbon and hydrogen content. The stoichiometric formula is CH₂O. The molecular formula can be C₂H₄O₂, C₃H₆O₃, etc. However, the compound shows acidic properties (based on the red indicator paper). This fits C₂H₄O₂, i.e. CH₃COOH, acetic acid, which is an acid without reducing properties.

Solution:

From the thermal analysis results, the mass of water lost during heating was:

The mass fraction of water and anhydrous cobalt sulfate in the sample is calculated as:

To calculate the ratio of crystalline water to anhydrous sulfate, we divide the obtained values by molar masses (thus obtaining the molar ratio):

The formula of the compound is CoSO₄·6.41H₂O, meaning the cobalt(II) sulfate heptahydrate was partially “weathered”.

Solution:

We calculate the molar amounts corresponding to the given mass ratios:

To obtain whole-number ratios, we multiply all molar amounts by two:

The empirical formula of the compound is C₃H₆O.

Solution:

To determine the formula, we must calculate the molar mass of the compound. We assume that the ratio of the masses of the compound and hydrogen equals the ratio of their molar masses:

By rearranging, we calculate the unknown molar mass:

The calculated molar mass is used to determine the molar amounts of elements in one mole of the compound:

Substituting the respective values, we obtain:

The molecular formula of the compound is C₂H₂ – acetylene

Solution:

The percentage of oxygen must be calculated:

w(O) = 100 – 77.73 = 22.27%

Next, calculate the corresponding molar amounts of the elements in the compound:

The molar amounts of iron and oxygen in the oxide are identical, 1.392 mol, so their ratio is 1 : 1.

Solution:

We calculate the molar amounts of the elements:

The stoichiometric formula is CH₂O, the molecular formula has doubled coefficients, corresponding to oxalic acid H₂C₂O₄.

Solution:

The compound has the stoichiometric formula H_xAs_yO_z. The indices x, y, and z are determined as follows:

The ratios are adjusted to small whole numbers x ∶ y ∶ z = 4 ∶ 2 ∶ 7

The stoichiometric formula of the compound is H₄As₂O₇ – pyroarsenic acid.