$500g$ toothpaste sample has $0.2g$ fluoride ion concentration. What is the concentration of fluoride in terms of $ppm$ level?

Hint: $ppm$ is defined as parts per million. This unit is used when the amount of substance is very small. It is defined as the amount of solute present in one million parts of the given substance. It can also be defined as the milligrams of the solute present per liter of the given substance.

Formula used: Concentration in $ppm = \dfrac{{{\text{mass of solute}}}}{{{\text{mass of parent substance}}}} \times {10^6}$

Complete step by step answer:
In this question we have given $500g$ of toothpaste. In this toothpaste $0.2g$ of fluoride ions is present. We have to find the concentration of this fluoride ion in $ppm$. $ppm$ is defined as parts per million. It is defined as the amount of solute present in one million parts of the given substance. In this question the solute is fluoride ion and the parent substance is toothpaste. According to question,
Mass of parent substance$ = 500g$
Mass of solute$ = 0.2g$
Substituting these values in the formula,
Concentration in $ppm = \dfrac{{{\text{mass of solute}}}}{{{\text{mass of parent substance}}}} \times {10^6}$
Concentration in $ppm = \dfrac{{0.2}}{{500}} \times {10^6}$
Solving this we get,
Concentration in $ppm = 400$
concentration of fluoride ions in $ppm$ is $400ppm$.

Additional Information:
like $ppm$, $ppb$ is also used to represent concentration. $ppb$ is defined as parts per billion. $ppb$ is the amount of solute present per billion parts of the parent substance. Like $ppm$, $ppb$ is also used when concentration of the substance is very less.

Note:
$ppm$ is the way to define very small concentrations.
Molarity is defined as the moles of the solute present per liter of the solution. Molarity is one of the ways of representing concentration.
Molality is defined as the moles of solute present per kilogram of the solvent.