Chapter 3, Problem 75A

Interpretation Introduction

Interpretation:

The mass percent of oxygen in magnesium oxide needs to be determined.

Concept introduction:

For any compound, the mass percent of any element present in it can be calculated as follows:

$\%m=\frac{m_{element}}{m_{compound}}\times 100\%$

Answer to Problem 75A

39.70%

Explanation of Solution

The mass of magnesium is 10.57 g and that of oxygen is 6.96 g. It is given that both the reactants completely react to form magnesium oxide.

According to the law of conservation of energy, total mass of reactants will be equal to products. In this case,

$m_{Mg}+m_{O_2}=m_{MgO}$

Putting the values,

$m_{MgO}=\left(10.57+6.96\right)\text{ g}=\text{17}\text{.53 g}$

Now, the mass of oxygen in the magnesium oxide is 6.96 thus, the mass percent of oxygen can be calculated as follows:

$\%m_O=\frac{m_O}{m_{MgO}}\times 100\%$

Putting the values,

$\begin{array}{l}\%m_O=\frac{6.96}{17.53}\times 100\%\\ =39.70\%\end{array}$

Conclusion

Therefore, the mass percent of oxygen is $39.70\%$.