Question

calculate percent by mass/percent composition.

1. a. na₂so₄

b. k₃po₄

c. al₂(co₃)₃

Explanation:

Part a: $\boldsymbol{\ce{Na2SO4}}$

Step1: Find molar mass of $\ce{Na2SO4}$

Molar mass of $\ce{Na}$: $23\ \text{g/mol}$, $\ce{S}$: $32\ \text{g/mol}$, $\ce{O}$: $16\ \text{g/mol}$.
Molar mass $= 2\times23 + 32 + 4\times16 = 46 + 32 + 64 = 142\ \text{g/mol}$.

Step2: Percent of $\ce{Na}$

$\% \ce{Na} = \frac{2\times23}{142} \times 100\% = \frac{46}{142} \times 100\% \approx 32.4\%$.

Step3: Percent of $\ce{S}$

$\% \ce{S} = \frac{32}{142} \times 100\% \approx 22.5\%$.

Step4: Percent of $\ce{O}$

$\% \ce{O} = \frac{4\times16}{142} \times 100\% = \frac{64}{142} \times 100\% \approx 45.1\%$.

Part b: $\boldsymbol{\ce{K3PO4}}$

Step1: Find molar mass of $\ce{K3PO4}$

Molar mass of $\ce{K}$: $39\ \text{g/mol}$, $\ce{P}$: $31\ \text{g/mol}$, $\ce{O}$: $16\ \text{g/mol}$.
Molar mass $= 3\times39 + 31 + 4\times16 = 117 + 31 + 64 = 212\ \text{g/mol}$.

Step2: Percent of $\ce{K}$

$\% \ce{K} = \frac{3\times39}{212} \times 100\% = \frac{117}{212} \times 100\% \approx 55.2\%$.

Step3: Percent of $\ce{P}$

$\% \ce{P} = \frac{31}{212} \times 100\% \approx 14.6\%$.

Step4: Percent of $\ce{O}$

$\% \ce{O} = \frac{4\times16}{212} \times 100\% = \frac{64}{212} \times 100\% \approx 30.2\%$.

Part c: $\boldsymbol{\ce{Al2(CO3)3}}$

Step1: Find molar mass of $\ce{Al2(CO3)3}$

Molar mass of $\ce{Al}$: $27\ \text{g/mol}$, $\ce{C}$: $12\ \text{g/mol}$, $\ce{O}$: $16\ \text{g/mol}$.
Molar mass $= 2\times27 + 3\times(12 + 3\times16) = 54 + 3\times(12 + 48) = 54 + 3\times60 = 54 + 180 = 234\ \text{g/mol}$.

Step2: Percent of $\ce{Al}$

$\% \ce{Al} = \frac{2\times27}{234} \times 100\% = \frac{54}{234} \times 100\% \approx 23.1\%$.

Step3: Percent of $\ce{C}$

$\% \ce{C} = \frac{3\times12}{234} \times 100\% = \frac{36}{234} \times 100\% \approx 15.4\%$.

Step4: Percent of $\ce{O}$

$\% \ce{O} = \frac{9\times16}{234} \times 100\% = \frac{144}{234} \times 100\% \approx 61.5\%$.

Answer:

s:
a. $\ce{Na}$: ~32.4%, $\ce{S}$: ~22.5%, $\ce{O}$: ~45.1%
b. $\ce{K}$: ~55.2%, $\ce{P}$: ~14.6%, $\ce{O}$: ~30.2%
c. $\ce{Al}$: ~23.1%, $\ce{C}$: ~15.4%, $\ce{O}$: ~61.5%