The Concept:
Mass Percent and Mole Solutions
Chemists often work with a physical mixture that is a specific type of solid dissolved in a liquid (such as salt dissolved in water). This combination is called a solution, and it allows chemists to accurately quantify their experiments. Solutions come it two major types, Mass Percent and Mole. Solutions also have two major parts to them, the solute and the solvent. The solute can be defined as the substance in the smaller proportion. The solvent is the substance in the larger proportion.
Mass Percent Solutions
A mass percent solution is based totally on the mass of the materials placed into the solution. Both the amount of solute to be used, and the amount of liquid to be used are weighed in some way before the solution is made, to insure a correct mixture. For example if a student weighed out 100.0grams of table salt (NaCl) and 900.0 grams of water, and mixed them together, the student would have a 10% table salt (NaCl) solution. The equation below shows the calculation. To get the mass% simply divide the mass of the solute (the table salt) by the total mass of everything in the solution (the table salt + the water) and multiply by 100. The equation below shows the calculation.
| Mass % |
= |
100 |
x |
 |
mass of
solute |
 |
| mass of solute + mass of solvent |
| Solute = 100.0 grams of NaCl |
Solvent = 900.0 grams of H2O |
|
| Mass % |
= |
100 |
x |
|
100.0 g NaCl |
|
| 100.0 g NaCl + 900.0 g of H2O |
| Mass % |
= |
100 |
x |
|
100.0 g |
|
| 1000.0 g |
Mole Solutions
Mole solutions have a solute and solvent component just like the mass percent solution. This is one of the only similarities between the two solutions though. Mole solutions are different from mass percent solutions both in how they are calculated, and how they are made. Mole solutions also require that the solute also be weighed, and this is the first step in creating any solution of this type. The next step is to take this solute and add solvent to it until a desired volume is achieved. This is the first major difference in the solutions as a total solution volume, rather than a total solution mass is desired.
The other major difference in the solutions is the units they are reported in. Mass percent solutions don’t have any extra units on them. The calculation performed to determine them involves dividing the mass of one material by the combined mass of the total solution. This causes the mass units to drop out of the final answer, leaving only a percentage since the calculation is multiplied by 100. Mole solutions do have physical units attached to them, which are moles and liters. The moles refer to how many moles of solute are used to create the solution. The liter’s refers to what final volume of the solution. So if we take the above example using the 100.0grams of table salt (NaCl) to make a solution, we must first weigh out the 100.0 grams of NaCl. Since the units on the mole solution for the solute are in moles we must convert this mass into moles by dividing by the molecular weight of NaCl. Doing this will cause the gram units to disappear and leave only moles.
|
100.0 g NaCl |
|
= |
1.71 moles of NaCl |
| 58.44 g / mole |
Knowing the moles of solute in the solution will now allow us to correctly make the type of mole solution we want. All we have to do is add solvent to the solute until the correct total solution volume is achieved. Note again that we are not measuring the volume of the solvent by itself, rather the combined volume of the solute and the solvent. So if enough solvent (water in this case) was added to the NaCl so the final total volume was 1.00Liter, a solution of 1.71 moles of NaCl per 1.00Liters would be created. This is also called a 1.71 Molar solution. By dividing the total moles by the final volume you can get the molarity of any solution.
| Molarity |
= |
|
moles of solute |
|
| total volume of solution (in Liters) |
| Molarity |
= |
|
1.71 moles of NaCl |
|
| 1.00 L of solution |
| Molarity |
= |
1.71 Molar solution |
So for example if I added enough water to get to a final solution volume of 2.00 Liters to the 100.0grams of NaCl, I would end up with a different solution molarity, half of what the above calculation is.
| Molarity |
= |
|
1.71 moles of NaCl |
|
| 2.00 L of solution |
| Molarity |
= |
0.86 Molar solution |
Again note that the volume of the solvent added is not used in this calculation, only the total solution volume. If 1.00L of solvent was added to the 100.0grams of NaCl the combined volume would be greater than 1.00L because the NaCl also takes up volume even when dissolved in water! This is why the final solution volume after the two are mixed is used.
