How to Prepare a 40% Solution by Weight
Chemists use weight percent as one method of expressing the concentration of a solution. In this case, a "solution" refers to a mixture of two miscible liquids or a solid dissolved in a liquid. By definition, the component present in the smaller amount represents the solute, and the greater component represents the solvent. The weight percent -- sometimes called the "weight/weight percent" or abbreviated "w/w%" -- is given by the formula weight percent = (mass of solute) / (mass of solution) * 100.
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Things You'll Need
- Balance capable of measuring to 1 gram
- Glass beaker or flask
- Calculator
- 100-mL graduated cylinder
Instructions
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Dissolving a Solid in a Liquid
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1
Weigh out 40 g of solid on a balance and transfer it to a glass container, such as a beaker or Erlenmeyer flask.
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2
Refer to an online chemical reference book, such as that provided in the Resources, to determine the density of the liquid solvent. If you are preparing an aqueous solution in which water serves as the solvent, then its density is 1.00 grams per milliliter, or 1.00 g/mL. Any other liquid will require density data.
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3
Calculate the volume, V, of liquid required to exhibit a mass of 60 grams by dividing 60 by the density of the liquid. In the case of water, this would be 60 milliliters. If you used a different solvent, such as acetone, which exhibits a density of 0.788 g/mL, then V = 60 / 0.788 = 76 mL.
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4
Measure out the volume of solvent determined in Step 3 using a 100-mL graduated cylinder and add this volume of solvent to the glass container with the solid. Swirl the container until the solvent completely dissolves the solid. If the solid does not dissolve, then gently heat the container while stirring the solution with a glass stirring rod to facilitate dissolution. When the solid has fully dissolved, the solution will contain 40 grams of solid and exhibit a total mass of 100 grams. It therefore represents a (40 g) / (100 g) * 100 = 40 percent solution by weight.
Mixing Two Liquids
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5
Refer to a reference source, such as that provided in the Resources, to determine the densities of the two liquids.
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6
Calculate the volume, V, of the solute necessary to achieve a mass of 40 grams: V = 40 g / d, where d represents the density of the solute in g/mL. For methanol, for example, with a density of 0.79 g/mL, V = 40 / 0.79 = 51 mL.
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7
Determine the volume of the solvent necessary to achieve a mass of 60 grams. In this case V = 60 g / d. For water, with a density of 1.00 g/ mL, V = 60 / 1.00 = 60 mL.
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8
Measure out the volumes of the solute and solvent separately in a 100-mL graduated cylinder and add each of them to a beaker or flask. Swirl the flask to mix the contents. The flask will then contain a 40-percent solution by weight.
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1
Tips & Warnings
The instructions provide 100 milliliters of solution, but they are directly scalable. Thus, if you require a larger volume, such as 1000 mL, simply multiply all masses and volumes by 10.
Most organic solvents, such as acetone and methanol, are highly flammable. Work away from any possible ignition sources. The use of eye protection and gloves is always recommended when working with chemicals.
