How to calculate solution concentration
Solution concentration can be expressed in many ways. However, here, we will discuss solution concentration in Percent by mass
What’s solution concentration?
Solution concentration is the amount of solute dissolved in a given amount of solution or solvent. Generally, we can express solution concentration mathematically as:
Solution concentration = amount of solute/amount of solution or solvent
The solute is the chemical in lesser amounts, while the solvent is the chemical in larger amounts.
What’s solution concentration in percent by mass?
Percent by mass (% m) is the mass of solute divided by the mass of solution multiplied by 100. Mathematically, % by mass = (mass of solute in grams/mass of solution in grams) x 100.
Notice! Mass of solution = mass of solute in grams plus mass of solvent in grams.
Since the grams cancel out, the final unit will be in percent. For example, a 20.0 percent sodium chloride (NaCl) solution will contain 20.0 g of NaCl in 100 g of solution (20 g/100 g). Since mass of solution is equal to 100 g, we can get the mass of solvent by subtracting 20 g of solute from the 100 g of solution (100 g – 20 g = 80 g of solvent). So, if water was used as our solvent, our mass of water will be 80 g. This means that the 20 g of NaCl/80 g of water can be used as a conversion factor to convert from mass of solute to mass of solvent and vice versa.
How many grams of NaCl must be added to 1000 g
of water to prepare a 20.0 percent NaCl solution.
The 20 percent NaCl solution means 20 g of NaCl/100 g of solution (read as 20 g of NaCl in a 100 g of solution). Since the question demands how many grams of NaCl must be added to 1000 g of water, we have to figure out the amount of water in the 20 percent of solution so that we can set up the correct conversion factor. To get that, we simply subtract 20 g of NaCl from the 100 g of solution (100 g – 20 g = 80 g of water).
Now, our conversion factor is: 20 g NaCl/80 g water.
To figure out how many grams of NaCl must be added to 1000 g, we simply multiply the 1000 g of water by 20 g NaCl/80 g water. If this calculation confuses you, click here to read more about conversion factors.
Thus, 1000 g H2O x 20 g NaCl/80 g H2O = 250 g. Therefore, 250 g of NaCl must be dissolved in 1000 g of water to make a 20 % NaCl solution.
Here is a summary of the calculation:
To learn how to calculate solution concentration in Molarity, click here