The percentage of elements

Tutorial 4

Reminder The atomic (molar) mass of an element ($ M $) is the mass (in grams) of one mole of atoms of this element! The atomic (molar) masses are found in the → periodic table The molar mass of a substance ($ M $) is the mass (in grams) of one mole of molecules or structural units of that substance! The molar masses of substances are calculated by summing the atomic masses of the → periodic table . Calculation of the percentage of an element in a substance from the formula of the substance 1) Calculate the molar mass $ M $ 2) Calculate the sum $ S $ of the atomic masses of this element (eg if the element intervenes with the coefficient 3, multiply its atomic mass by 3) 3) $ \% = \frac{S \cdot 100}{M} $

$ 50 \; g $ of a sample of a carbon, oxygen and hydrogen compound contain: $ 20 \; g \; C $ and $ 26.7 \; g \; O $ Give all possible molecular formulas!

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Mass of $ H $ in the sample ?

$m_H$ $=$ $50-20-26.7$ $=$ $3.3\;g$

Transform the masses into moles!

$n_{H}=\frac{3.3}{1}=3.3$ $n_{C}=\frac{20}{12}=1.7$ $n_{O}=\frac{26,7}{16}=1.7$

Compare and make a conclusion!

It can be seen that the numbers of moles are in the ratio 2: 1: 1 for H/C/O Possible formulas: $CH_2O$ $C_2H_4O_2$ $C_3H_6O_3$ $C_4H_8O_4$ .... $C_nH_{2n}O_n$