Question

3. The same well water contains dissolved calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K) at 5.0, 1.0, 136 and 1.2 mg/L, respectively. Determine these concentrations in units of mol/L.

Answer 1

Answer: The concentration of calcium, magnesium, sodium and potassium are
`1.25* 10^(-4)mol/L`,
`4.20* 10^(-5)mol/L`,
`5.91* 10^(-3)mol/L` and
`3.08* 10^(-5)mol/L` respectively

Step-by-step explanation:

To convert the mass from milligrams to grams, we use the conversion factor:

1 g = 1000 mg

To convert the given concentration fro grams to moles, we use the equation:

`\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}` .....(1)

• For Calcium:

Concentration given = 5.0 mg/L =
`5.0* 10^(-3)g/L`

We know that:

Molar mass of calcium element = 40 g/mol

Putting values in equation 1, we get:

`\text{Concentration in mol/L}=(5.0* 10^(-3)g/L)/(40g/mol)\\\\\text{Concentration in mol/L}=1.25* 10^(-4)mol/L`

• For Magnesium:

Concentration given = 1.0 mg/L =
`1.0* 10^(-3)g/L`

We know that:

Molar mass of magnesium element = 24 g/mol

Putting values in equation 1, we get:

`\text{Concentration in mol/L}=(1.0* 10^(-3)g/L)/(24g/mol)\\\\\text{Concentration in mol/L}=4.20* 10^(-5)mol/L`

• For Sodium:

Concentration given = 136 mg/L =
`136* 10^(-3)g/L`

We know that:

Molar mass of sodium element = 23 g/mol

Putting values in equation 1, we get:

`\text{Concentration in mol/L}=(136* 10^(-3)g/L)/(23g/mol)\\\\\text{Concentration in mol/L}=5.91* 10^(-3)mol/L`

• For Potassium:

Concentration given = 1.2 mg/L =
`1.2* 10^(-3)g/L`

We know that:

Molar mass of potassium element = 39 g/mol

Putting values in equation 1, we get:

`\text{Concentration in mol/L}=(1.2* 10^(-3)g/L)/(39g/mol)\\\\\text{Concentration in mol/L}=3.08* 10^(-5)mol/L`

Hence, the concentration of calcium, magnesium, sodium and potassium are
`1.25* 10^(-4)mol/L`,
`4.20* 10^(-5)mol/L`,
`5.91* 10^(-3)mol/L` and
`3.08* 10^(-5)mol/L` respectively