Question

The two isotopes of potassium with significant abundance in nature are 39K asotopic mass 38.9637 amu, 932.58%) and 41K Osotopic mass 40.9618 amu, 6.730%). Fluorine has only one naturally occurring isotope, 19F (isotopic mass 18.9984 amu). Calculate the formula mass of potassium fluoride.

Answer 1

Answer:

$\boxed{\text{58.0919 u}}$

Explanation:

1. Atomic mass of K

To get the atomic mass of K, you calculate the weighted average of the isotopic masses.

That is, you multiply the atomic mass of each isotope by a number representing its relative importance (e.g., its percent of the total).

Set up a table for easy calculation:

$\begin{array}{ccrcr}\textbf{Atom} & \textbf{Mass/u} &\textbf{Percent} & \textbf{Calculation}& \textbf{Result/u}\\^{39}\text{K}& 38.9637 & 93.258 & 38.9637 \times 0.932 58 & 36.3368\\^{41}\text{K}& 40.9618 & 6.730 & 40.9618 \times 0.06730& 2.7567\\& & \text{TOTAL } = &\textbf{39.0935}\\\end{array}$

2. Formula mass of KF

$\begin{array}{rcl}\text{K} & = & \text{39.0935 u}\\\text{F} & = & \text{18.9984 u}\\\text{Total} & = & \textbf{58.0919 u}\\\end{array}\\\text{The formula mass using only the two isotopes of K is \boxed{\textbf{58.0919 u}} }$