Question

1. a sample has a nitrate concentration of 5.42 mg/l (5.42 ppm no₃⁻). calculate its molar concentration (in moles/l). (this primarily involves unit conversions, including using the molar mass of nitrate to convert mass to moles.)

Explanation:

Step1: Determine molar mass of $NO_3^-$

The molar mass of N is approximately 14 g/mol and of O is approximately 16 g/mol. For $NO_3^-$, molar mass $M=(14 + 3\times16)\text{ g/mol}=62\text{ g/mol}=62000\text{ mg/mol}$.

Step2: Convert mass - concentration to molar - concentration

We know that molar - concentration $c=\frac{\text{mass - concentration}}{\text{molar mass}}$. Given mass - concentration is 5.42 mg/L. So, $c=\frac{5.42\text{ mg/L}}{62000\text{ mg/mol}}$.

$$c = 8.74\times10^{-5}\text{ mol/L}$$

Answer:

$8.74\times10^{-5}\text{ mol/L}$