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# A solution contains 40. mEq/L of Cl- and 15 mEq/L of HPO4 2-. If Na+ is the only cation in the solution, what is Na+ concentration, milliequivalents per liter?

##"55 mEq/L"##

The idea here is that you need to use the fact that a solution is **electrically neutral** to determine how many milliequivalents of sodium cations are needed to **balance** the milliequivalents of the two anions.

The two anions present in solution are

- the chloride anion, ##"Cl"^(-)##, which has a concentration of ##"40. mEq/L"##
- the hydrogen phosphate anion, ##"HPO"_4^(2-)##, which has a concentration of ##"15 mEq/L"##

As you know, an **equivalent** for an ion is calculated by multiplying the number of moles of that ion by its **valence**.

In this case, every liter of solution will contain ##"40. mEq"## of chloride anions and ##"15 mEq"## of hydrogen phosphate anions, which means that the total number of milliequivalents for the anions will be

##"no. of mEq anions" = "40. mEq" + "15 mEq" = "55 mEq"##

You know that a solution is electrically neutral, so

##color(blue)("no. of mEq anions " = " no. of mEq cations")##

Because sodium is the only cation present in this solution, its number of milliequivalents **must be equal** to the total number of milliequivalents for the anions.

This means that the solution will contain ##"55 mEq"## of sodium for every liter of solution, which is equivalent to a concentration of

##["Na"^(+)] = color(green)("55 mEq/L")##