1. A graduate measured out a 100. mL hard water sample and titrated it with 37.6 mL of 0.0100 M EDTA solution for a total hardness endpoint, and 29.3 mL of 0.0100 M EDTA solution for a calcium endpoint. Calculate the molarity with respect to magnesium and calcium. Please calculate the mg/L Ca and mg/L Mg of the unknown
2. Explain the different conductivity values expected from both a rain water sample and an oil field brine water sample
1. To calculate the molarity with respect to magnesium and calcium, we can use the titration data provided.
Step 1: Calculate the number of moles of EDTA used for each endpoint.
For the total hardness endpoint (EDTA reaction with both magnesium and calcium):
moles of EDTA = volume of EDTA solution (L) × molarity of EDTA solution (mol/L)
moles of EDTA = 37.6 mL × 0.0100 mol/L = 0.376 mmol
For the calcium endpoint (EDTA reaction with only calcium):
moles of EDTA = 29.3 mL × 0.0100 mol/L = 0.293 mmol
Step 2: Calculate the number of moles of calcium:
moles of calcium = moles of EDTA at calcium endpoint - moles of EDTA at total hardness endpoint
moles of calcium = 0.293 mmol - 0.376 mmol = -0.083 mmol
(Note: negative value means excess EDTA was used, implying all calcium ions reacted)
Step 3: Calculate the molarity of calcium:
molarity of calcium = moles of calcium / volume of water sample (L)
Given that the water sample volume is 100 mL = 0.100 L, we can convert the moles of calcium to molarity:
molarity of calcium = -0.083 mmol / 0.100 L = -0.83 mmol/L
Since concentration cannot be negative, we will assume the excess EDTA is from magnesium and all calcium has reacted.
Step 4: Calculate the number of moles of magnesium:
moles of magnesium = moles of EDTA at total hardness endpoint - moles of calcium
moles of magnesium = 0.376 mmol - 0.293 mmol = 0.083 mmol
Step 5: Calculate the molarity of magnesium:
molarity of magnesium = moles of magnesium / volume of water sample (L)
molarity of magnesium = 0.083 mmol / 0.100 L = 0.83 mmol/L
To calculate the mg/L Ca and mg/L Mg of the unknown, we need to convert the molarities to mg/L.
Step 6: Calculate the mass of calcium and magnesium:
mass of calcium = molarity of calcium × molar mass of calcium
mass of magnesium = molarity of magnesium × molar mass of magnesium
(Note: The molar mass of calcium is 40.08 g/mol, and the molar mass of magnesium is 24.31 g/mol.)
Step 7: Convert the mass to mg:
mg/L Ca = mass of calcium × 1000 mg/g
mg/L Mg = mass of magnesium × 1000 mg/g
By following these steps, you should be able to calculate the molarity with respect to magnesium and calcium, as well as the mg/L Ca and mg/L Mg of the unknown sample.
2. Rainwater is typically a low-conductivity sample due to its low concentration of dissolved solids. It is created through the condensation of water vapor in the atmosphere, and as it falls, it generally comes into contact with the atmosphere, which contains low levels of ions and other dissolved substances.
In contrast, oil field brine water is expected to have a high conductivity due to its high concentration of dissolved salts. Brine water is found naturally in oil reservoirs and is often present in oil wells. As water is extracted along with the oil, it carries dissolved ions and salts from the surrounding rock formations. These dissolved solids significantly increase the conductivity of the brine water.
Therefore, rainwater will have a lower conductivity compared to oil field brine water due to the differences in their concentrations of dissolved solids.
1. To find the molarity of magnesium and calcium in the water sample, we need to use the titration data provided. Let's break down the process step by step:
Step 1: Determine the number of moles of EDTA solution used for the total hardness endpoint.
To do this, we multiply the volume of the EDTA solution used (37.6 mL) by its molarity (0.0100 M):
moles of EDTA = 37.6 mL * 0.0100 M = 0.376 moles
Step 2: Calculate the number of moles of calcium in the water sample.
Since 29.3 mL of the same EDTA solution was used for the calcium endpoint, we can subtract this amount to determine the moles of only magnesium:
moles of magnesium = moles of EDTA (total hardness) - moles of EDTA (calcium) = 0.376 moles - 0.293 moles = 0.083 moles
Step 3: Convert moles to grams.
To find the mass of calcium and magnesium, we need to multiply the number of moles by their respective molar masses. The molar mass of calcium is 40.08 g/mol, and the molar mass of magnesium is 24.31 g/mol:
mass of calcium = 0.083 moles * 40.08 g/mol = 3.33 g
mass of magnesium = 0.083 moles * 24.31 g/mol = 2.02 g
Step 4: Calculate the concentration in mg/L.
Finally, we can convert the mass of calcium and magnesium to mg/L. Since 1 g/L is equivalent to 1000 mg/L:
concentration of calcium (mg/L) = mass of calcium (g) * 1000 = 3.33 g * 1000 = 3330 mg/L
concentration of magnesium (mg/L) = mass of magnesium (g) * 1000 = 2.02 g * 1000 = 2020 mg/L
Therefore, the molarity with respect to magnesium is 0.083 M, and the molarity with respect to calcium is also 0.083 M. The unknown sample has a concentration of 3330 mg/L of calcium and 2020 mg/L of magnesium.
2. Rainwater and oil field brine water samples have different conductivity values due to their distinct compositions.
Rainwater:
Rainwater is usually pure, with very low levels of dissolved minerals and ions. As a result, it has low electrical conductivity. In areas where the rainwater is exposed to pollutants or contaminants, the conductivity can increase due to the presence of dissolved impurities such as salts, acids, or other substances from the environment.
Oil field brine water:
Oil field brine water is a byproduct of oil extraction. It often contains a high concentration of dissolved salts, such as sodium chloride, calcium chloride, and magnesium chloride. These dissolved salts significantly increase the electrical conductivity of the water. Therefore, oil field brine water has a higher conductivity compared to rainwater.
In summary, rainwater generally has low electrical conductivity due to its low mineral content, while oil field brine water has high conductivity due to the presence of dissolved salts.