Question

STEM 11
1.How many moles of carbon dioxide does 250 grams of the substance have? How many molecules of CO2 are there?
2. Determine the percent composition of each element in the compound in tartaric acid (C4H6O6)
3. What are the empirical and molecular formula of a compound that is 40.27% potassium, 26.78%
chromium, and 32.95% oxygen?
4. Balance the following equations
1. ____ C + ____ O2 → ___ CO
2. ____ Mg + ____ O2 → ____ MgO
3. ____ H2O2 → ____ H2O + ____ O2
4. ____ CH4 + O2 → ____ CO2 + ____ H2O
5. ____ N2O5 → _____ N2O4 + _____ O2

Answer 1

The number of molecules can be obtained from Avogadro's number. A balanced equation contains the same number of atoms on both sides of the reaction equation.

1) Number of molecules = mass/molar mass = 250 grams/44 g/mol = 5.68 moles

Given that;

1 mole contains 6.02 ×  10^23 molecules of CO2

5.68 moles contains 5.68  × 6.02 ×  10^23/1

= 3.42 ×  10^24 molecules of CO2

2) The molar mass of the element is; 4(12) + 6(1) + 6(16) = 48 + 6 + 96 = 150 g/mol

Percentage by mass of C = 4(12)/150  × 100 = 32%

Percentage by mass of H = 6(1) /150  × 100 = 4 %

Percentage by mass of O = 6(16)/150  × 100 = 64 %

3) Divide the percentage by mass by the relative atomic mass of each element;

K - 40.27%/39    Cr - 26.78%/52    O - 32.95%/16

K  - 1.03              Cr - 0.52               O - 2.06

Divide through by the lowest ratio

K - 1.03/0.52      Cr - 0.52/0.52      O - 2.06/0.52

K - 2                   Cr - 1                       O - 4

The required formula is K2CrO4

4) The balanced equation are;

2C + O2 -----> 2CO

2Mg + O2 -----> 2MgO

2H2O2 ----> 2H2O + O2

CH4 + 2O2 ----> CO2 + 2H2O

2N2O5 -----> 2N2O4 + O2

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