Chapter 3.4, Problem 29SSC

Interpretation Introduction

Interpretation: The mass percent of hydrogen in water and the mass percent of oxygen in water needs to be determined.

Concept Introduction: The mass percentage is defined as the percentage of the component present in it. The mass percentage of the element is the mass of element divided by the total mass of the compound and multiplied by 100.

  $\text{Mass percentage =}\frac{\text{mass}\text{of}\text{element}}{\text{mass}\text{of}\text{total}\text{compound}}\text{×100}$

Answer to Problem 29SSC

The mass percentage of the element hydrogen is $11.2\%$ and the mass percentage of the element oxygen is $88.8\%$ in the formed compound.

Explanation of Solution

Given information:

Two equivalents of H, its molar mass is $M_H=1.0079\text{g/mol}$

One equivalent of O, its molar mass is $M_o=15.999\text{g/mol}$

In a water molecule, the two elements of hydrogen and one element of oxygen is present.

The mass of hydrogen for one element is $1.0079$ and for oxygen of mass is $15.999$

  $\text{Mass percentage =}\frac{\text{mass}\text{of}\text{element}}{\text{mass}\text{of}\text{total}\text{compound}}\text{×100}$

Mass of compound water, ${\text{H}}_{\text{2}}\text{O}=\text{1}\text{.0079 g + 15}\text{.999 g = 18}\text{.015 g}$

Percentage by mass of an element, $\text{H}=\frac{\text{2×1}\text{.0079 g}}{\text{18}\text{.015 g}}\text{×100}\text{=}\text{11}\text{.2\%}$

There are $2$ elements of hydrogen present in a single water molecule, so for the calculation of the percentage of the mass of only one hydrogen, it should be multiplied with $2$ .

  $\text{pecentage}\text{by mass of element O}=\frac{\text{15}\text{.999 g}}{\text{18}\text{.015 g}}\text{×100}\text{=}\text{88}\text{.8\%}$

Conclusion

From the mass of an element and the mass of the compound, the percentage by mass of elements is calculated in the formed compound. So, the percentage of the mass of hydrogen is $11.2\%$ and oxygen is $88.8\%$ .