Question

a student dissolved 0.515 mol of magnesium chloride in enough water to make 225 ml of solution. what is the molar concentration of the magnesium ion in solution?
2.29 mol/l
0.00229 mol/l
4.58 mol/l
0.44 mol/l

the concentration of fluoride ions, f−, in municipal drinking water cannot exceed 1.5 ppm. what is the maximum mass of fluoride ions that can be dissolved in 500 ml of water?
0.75 mg
1.5 mg
750 mg
3.0 mg

Explanation:

Step1: Convert volume to liters

$225\ \text{mL} = \frac{225}{1000} = 0.225\ \text{L}$

Step2: Find $MgCl_2$ molarity

$\text{Molarity of } MgCl_2 = \frac{0.515\ \text{mol}}{0.225\ \text{L}} \approx 2.29\ \text{mol/L}$

Step3: Relate to $Mg^{2+}$ concentration

1 mole $MgCl_2$ gives 1 mole $Mg^{2+}$, so $\text{[Mg}^{2+}] = 2.29\ \text{mol/L}$

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Step1: Define ppm for aqueous solution

1 ppm = 1 mg/L (for dilute water solutions)

Step2: Calculate mass for 500 mL

$500\ \text{mL} = 0.5\ \text{L}$, so $\text{Mass} = 1.5\ \text{mg/L} \times 0.5\ \text{L} = 0.75\ \text{mg}$

Answer:

1. 2.29 mol/L
2. 0.75 mg