To find the percent composition of each element in a compound, we follow these steps:
1.
Find the atomic mass of each element from the periodic table.
2.
Calculate the total molar mass of the entire compound by adding up the masses of all atoms.
3.
Divide the total mass of each specific element by the total molar mass of the compound.
4.
Multiply by 100 to get the percentage.
Here are the step-by-step calculations for each problem on the worksheet:
1) $\text{CuBr}_2$ (Copper(II) Bromide)
*
Atomic Masses:
* Copper (Cu): $63.55 \text{ g/mol}$
* Bromine (Br): $79.90 \text{ g/mol}$
*
Total Molar Mass:
$$63.55 + (2 \times 79.90) = 63.55 + 159.80 = 223.35 \text{ g/mol}$$
*
Percent Composition:
*
Cu: $(63.55 / 223.35) \times 100 = \mathbf{28.45\%}$
*
Br: $(159.80 / 223.35) \times 100 = \mathbf{71.55\%}$
2) $\text{NaOH}$ (Sodium Hydroxide)
*
Atomic Masses:
* Sodium (Na): $22.99 \text{ g/mol}$
* Oxygen (O): $16.00 \text{ g/mol}$
* Hydrogen (H): $1.01 \text{ g/mol}$
*
Total Molar Mass:
$$22.99 + 16.00 + 1.01 = 40.00 \text{ g/mol}$$
*
Percent Composition:
*
Na: $(22.99 / 40.00) \times 100 = \mathbf{57.48\%}$
*
O: $(16.00 / 40.00) \times 100 = \mathbf{40.00\%}$ *(Note: The image lists 39.99%, likely due to slightly different rounding of atomic masses, but 40.00% is standard)*
*
H: $(1.01 / 40.00) \times 100 = \mathbf{2.53\%}$ (rounded from 2.525%)
3) $(\text{NH}_4)_2\text{S}$ (Ammonium Sulfide)
*
Atomic Masses:
* Nitrogen (N): $14.01 \text{ g/mol}$
* Hydrogen (H): $1.01 \text{ g/mol}$
* Sulfur (S): $32.06 \text{ g/mol}$
*
Count the atoms: There are 2 N, 8 H ($4 \times 2$), and 1 S.
*
Total Molar Mass:
$$(2 \times 14.01) + (8 \times 1.01) + 32.06 = 28.02 + 8.08 + 32.06 = 68.16 \text{ g/mol}$$
*
Percent Composition:
*
N: $(28.02 / 68.16) \times 100 = \mathbf{41.11\%}$
*
H: $(8.08 / 68.16) \times 100 = \mathbf{11.85\%}$
*
S: $(32.06 / 68.16) \times 100 = \mathbf{47.04\%}$
4) $\text{N}_2\text{S}_2$ (Dinitrogen disulfide)
*
Atomic Masses:
* Nitrogen (N): $14.01 \text{ g/mol}$
* Sulfur (S): $32.06 \text{ g/mol}$
*
Total Molar Mass:
$$(2 \times 14.01) + (2 \times 32.06) = 28.02 + 64.12 = 92.14 \text{ g/mol}$$
*
Percent Composition:
*
N: $(28.02 / 92.14) \times 100 = \mathbf{30.41\%}$
*
S: $(64.12 / 92.14) \times 100 = \mathbf{69.59\%}$
5) $\text{KMnO}_4$ (Potassium permanganate)
*
Atomic Masses:
* Potassium (K): $39.10 \text{ g/mol}$
* Manganese (Mn): $54.94 \text{ g/mol}$
* Oxygen (O): $16.00 \text{ g/mol}$
*
Total Molar Mass:
$$39.10 + 54.94 + (4 \times 16.00) = 39.10 + 54.94 + 64.00 = 158.04 \text{ g/mol}$$
*
Percent Composition:
*
K: $(39.10 / 158.04) \times 100 = \mathbf{24.74\%}$
*
Mn: $(54.94 / 158.04) \times 100 = \mathbf{34.76\%}$
*
O: $(64.00 / 158.04) \times 100 = \mathbf{40.50\%}$
*(Note: The values in red on your worksheet appear to be rounded to whole numbers or simplified decimals, e.g., K=25%, Mn=35%, O=40%. The calculations above provide the more precise scientific values.)*
Final Answer:
1) Cu: 28.45%, Br: 71.55%
2) Na: 57.48%, O: 40.00%, H: 2.53%
3) N: 41.11%, H: 11.85%, S: 47.04%
4) N: 30.41%, S: 69.59%
5) K: 24.74%, Mn: 34.76%, O: 40.50%
Parent Tip: Review the logic above to help your child master the concept of percent composition worksheet answers.
