Question

A sample of river water at 20°C is analyzed and the following parameters are found

for it:
Phenolphthalein alkalinity = 5.8 x10-6 M, Total alkalinity = 2.8 × 10-5 M; pH = 8.5
Hardness 10.0 mg CaCO3/L; [Mg2+] = 3.8 x 105 M
Calculate the concentrations of [Ca²+], [CO32] and [HCO3] in this water sample.

Please explain in detail

Answer 1

The concentrations of [Ca²+], [CO32-], and [HCO3-] in the water sample are 1.0 x 10-4 M, 3.8 x 10-4 M, and -3.5 x 10-4 M, respectively.

Step-by-step explanation:

The concentrations of [Ca²+], [CO32-], and [HCO3-] in the water sample can be calculated using the given parameters and chemical reactions. Here are the steps:

1. From the given hardness count of 10.0 mg CaCO3/L, we can convert it to molar concentration of Ca²+ ions:
   10.0 mg CaCO3/L x (1 mol Ca²+/100.09 g CaCO3) x (1 L/1000 mL) = 1.0 x 10-4 M [Ca²+]
2. Since the sample is analyzed at 20°C, we can use the relationship between phenolphthalein alkalinity and carbonate species:
   [CO32-] = Phenolphthalein alkalinity - 2 x [Ca²+]
   [CO32-] = 5.8 x 10-6 M - 2 x 1.0 x 10-4 M = 3.8 x 10-4 M
3. Using the total alkalinity, we can find the concentration of bicarbonate ions:
   [HCO3-] = Total alkalinity - [CO32-]
   [HCO3-] = 2.8 x 10-5 M - 3.8 x 10-4 M = -3.5 x 10-4 M

Therefore, the concentrations of [Ca²+], [CO32-], and [HCO3-] in the water sample are 1.0 x 10-4 M, 3.8 x 10-4 M, and -3.5 x 10-4 M, respectively.