# How many grams of #NaCl# should be weighed to prepare 1 L of 20 ppm solution of #Na^+#?

##### 1 Answer

You need **0.05 g** of sodium chloride.

#### Explanation:

In order to determine how much sodium chloride must be dissolved in **1 L** of water, start from the definition of *parts per million*, **ppm**.

A concentration of **1 ppm** is equivalent to 1 part solute, in your case sodium chloride, for every **1 million** parts solvent, in your case water.

To get a solution's concentration in ppm, you multiply the ratio that exists between the mass of the solute and the mass of the water by **1 million**, or

This is the exact same approach you use when calculating *percentage*, the only difference being the fact that you need to multiply the ratio by 1 million, instead of by 100.

So, you can safely assume the density of water to be equal to

Sodium chloride will dissociate completely in aqueous solution to form sodium cations,

The important thing to notice here is that you have a

The concentration of the target solution (in ppm) will be

Plug in your values and solve for

This means that your solution must contain **0.02 g** of sodium cations,

The aforementioned mole ratio tells you that you must add the *same number of moles* of sodium chloride to the solution.

Now use sodium chloride's molar mass to determine how much you need

So, if you dissolve **0.05 g** of sodium chloride in **1 L** of water you'll get a 20-ppm