The concentrations of Ca and Na in a sample of river water are 13.4 mg/kg and 5.2 mg/kg. Express the concentration in molality
Molality id defined as (moles of a solute)/(kg of solvent). So the concentration of Ca in the river water would be (13.4 mg/kg) * (1/1000 g/mg) * (1/40.078 mol/g) = 3.34 x 10^-4 mol/kg
The key to solving any concentration problem is knowing what units you're starting with and what units you want to end with. I'll leave the Na for you to solve, but the solution is essentially the same. Try to make sure your units cancel out to the proper unit at the end.
To express the concentration in molality, we need to convert the given mass concentrations of Ca and Na into moles per kilogram.
Molality (m) is defined as the number of moles of solute per kilogram of solvent.
We can use the molar mass of each element to convert the given mass concentrations into moles.
The molar mass of Ca is 40.08 g/mol, and the molar mass of Na is 22.99 g/mol.
First, let's convert the given mass concentrations from mg/kg to g/kg:
Mass concentration of Ca = 13.4 mg/kg = 0.0134 g/kg
Mass concentration of Na = 5.2 mg/kg = 0.0052 g/kg
Next, let's calculate the number of moles of each element:
Moles of Ca = (0.0134 g/kg) / (40.08 g/mol) = 0.000334 mol/kg
Moles of Na = (0.0052 g/kg) / (22.99 g/mol) = 0.000226 mol/kg
Finally, we can express the concentrations in molality:
Molality of Ca = 0.000334 mol/kg
Molality of Na = 0.000226 mol/kg
Therefore, the concentration of Ca in the river water sample is approximately 0.000334 mol/kg, and the concentration of Na is approximately 0.000226 mol/kg.
To express the concentration in molality, we need to convert the mass concentrations of Ca and Na in the sample of river water into molar concentrations. Molality is the number of moles of solute per kilogram of solvent.
To calculate the concentration in molality, we need the molar mass of Ca and Na. The molar mass of Ca is 40.08 g/mol, and the molar mass of Na is 22.99 g/mol.
Let's start with Ca:
1. Convert the mass concentration of Ca from mg/kg to g/kg:
13.4 mg/kg = 13.4 × 10^(-3) g/kg
2. Convert the mass from g/kg to moles, using the molar mass of Ca:
Moles of Ca = (13.4 × 10^(-3) g/kg) / 40.08 g/mol
Next, let's calculate Na:
1. Convert the mass concentration of Na from mg/kg to g/kg:
5.2 mg/kg = 5.2 × 10^(-3) g/kg
2. Convert the mass from g/kg to moles, using the molar mass of Na:
Moles of Na = (5.2 × 10^(-3) g/kg) / 22.99 g/mol
Finally, calculate the molality of each ion:
Molality of Ca = Moles of Ca / kg of solvent
Molality of Na = Moles of Na / kg of solvent
Remember that the solvent in this case is the river water.
By following these steps, you can now express the concentrations of Ca and Na in molality.