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How do you find the concentration of a saline solution?

Published in Chemistry Calculations 4 mins read

The concentration of a saline solution can be found by calculating the amount of salt (solute) present in a given amount of solution (salt and water). Concentration can be expressed in different ways, including percent concentration, molarity, and molality.

Here's a breakdown of how to calculate the concentration using different methods:

1. Percent Concentration (Weight/Volume % w/v):

This is the most directly supported method based on the reference video.

  • Definition: Percent concentration (w/v) expresses the mass of solute (salt) in grams per 100 mL of solution.

  • Formula:

    % (w/v) = (Mass of solute (g) / Volume of solution (mL)) x 100

  • Example: If you dissolve 15g of salt in enough water to make 200 mL of saline solution:

    % (w/v) = (15g / 200 mL) x 100 = 7.5%

    This means the saline solution is a 7.5% (w/v) saline solution.

2. Molarity (M):

  • Definition: Molarity expresses the number of moles of solute per liter of solution.

  • Formula:

    Molarity (M) = Moles of solute / Liters of solution

  • Steps:

    1. Convert mass of salt to moles: Divide the mass of the salt (in grams) by its molar mass (grams/mole). The molar mass of NaCl (sodium chloride), common table salt, is approximately 58.44 g/mol.
    2. Convert volume of solution to liters: Divide the volume of the solution (in mL) by 1000.
    3. Calculate Molarity: Divide the moles of solute by the liters of solution.

  • Example: You dissolve 5.844g of NaCl in enough water to make 100 mL of solution.

    1. Moles of NaCl: 5.844 g / 58.44 g/mol = 0.1 moles
    2. Liters of solution: 100 mL / 1000 mL/L = 0.1 L
    3. Molarity: 0.1 moles / 0.1 L = 1 M

    This means the saline solution is a 1 M (1 molar) solution.

3. Molality (m):

  • Definition: Molality expresses the number of moles of solute per kilogram of solvent (water in this case). This is different from molarity, which uses the volume of solution.

  • Formula:

    Molality (m) = Moles of solute / Kilograms of solvent

  • Steps:

    1. Convert mass of salt to moles: Divide the mass of the salt (in grams) by its molar mass (grams/mole).
    2. Convert mass of water to kilograms: Divide the mass of the water (in grams) by 1000. (Assuming the density of water is approximately 1 g/mL, the mass of water in grams will be numerically equal to its volume in mL). Note that this is the mass of solvent (water) and not the solution. To find the mass of the solvent if given the mass of the solute and volume of the solution, you need to subtract the mass of the solute from the mass of the solution. If you are given the volume of the solution, you'll need to know its density to find its mass.
    3. Calculate Molality: Divide the moles of solute by the kilograms of solvent.

  • Example: You dissolve 5.844g of NaCl in 900g (0.9 kg) of water.

    1. Moles of NaCl: 5.844 g / 58.44 g/mol = 0.1 moles
    2. Kilograms of water: 900 g / 1000 g/kg = 0.9 kg
    3. Molality: 0.1 moles / 0.9 kg = 0.111 m

    This means the saline solution is a 0.111 m (0.111 molal) solution.

In summary, determining the concentration of a saline solution depends on the units you want to use. Percent concentration (w/v), molarity, and molality are common ways to express the amount of salt present in the solution.

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