Question

unit review - matter and change
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a 150 g sample is composed of 90 g of magnesium (mg) and 60 g of oxygen (o).
what is the percent by mass of o in the sample?
50%
40%
70%
60%

Explanation:

Step1: Recall mass percent formula

Percent by mass = $\frac{\text{Mass of component}}{\text{Total mass of sample}} \times 100\%$

Step2: Plug in given values

Substitute mass of O = 60 g, total mass = 150 g
$\frac{60}{150} \times 100\%$

Step3: Calculate the result

Simplify $\frac{60}{150}=0.4$, then $0.4 \times 100\% = 40\%$

Answer:

40%