Question

A solution is formed by mixing 15.2 g KOH into

Answer 1

Answer:

There are approximately $0.271\; \rm mol$ of formula units in that $\rm 15.2\; g$ of $\rm KOH$ (the solute of this solution.)

Explanation:

A solution includes two substances: the solute and the solvent. Note the solution here contains significantly more water than $\rm KOH$. Hence, assume that water is the solvent (as it is in many other solutions.)

The (molar) formula mass of $\rm KOH$ is necessary for finding the number of moles of

• One $\rm K$ atom,
• One $\rm O$ atom, and
• One $\rm H$ atom.

The formula mass of $\rm KOH$ will thus be the sum of:

• The mass of one mole of $\rm K$ atoms,
• The mass of one mole of $\rm O$ atoms, and
• The mass of one mole of $\rm H$ atoms.

On the other hand, the mass (in grams) of one mole of atoms of an element is (numerically) the same as its relative atomic mass. The relative atomic mass data can be found on most modern periodic tables.

Relative atomic mass data from a modern periodic table:

• $\rm K$: $39.098$.
• $\rm O$: $15.999$.
• $\rm H$: $1.008$.

For example, the relative atomic mass of $\rm K$ (potassium, atomic number $19$) is $39.098$ (3 sig. fig.) Hence, the mass of one mole of

The formula mass of $\rm KOH$ is the sum of these three masses:

$\begin{aligned}& M(\mathrm{KOH}) \\ &\approx 39.098 + 15.999 + 1.008 \\ &= 56.105\; \rm g \cdot mol^{-1}\end{aligned}$.

The number of moles of $\rm KOH$ formula units in this $15.2\; \rm g$ sample would be:

$\begin{aligned}n &= \frac{m(\mathrm{KOH})}{M(\mathrm{KOH})} \\ &\approx \frac{15.2\; \rm g}{56.105\; \rm g \cdot mol^{-1}} \approx 0.271\; \rm mol \end{aligned}$.