Run off worksheets.
Molarity and molality of solutions are explained in most standard chemistry texts, and taught in most standard chemistry courses. In biology, however, solutions are more often written as a percentage rather than as a molarity or molality. (This keeps the biologist from having to add up molecular masses and doing any complex math.) Thus, while molarity and molality are often covered in a chemistry class, solutions as a percentage is not normally taught.
The concept is very straightforward. A 1% solution means there is 1 gram of solute dissolved in 99 grams of solvent. Thus, the solute makes up 1% of the total final mass of the solution.
As an example, if it is desired to prepare 50.0 grams of a 2.0% NaCl solution the following calculations would be made.
Total mass of solute and solvent = 50.0 grams
Mass of solute (NaCl) = 50.0 g X 2% = 1.0 g
Mass of solvent (water) = 50.0 g - 1.0 grams = 49.0 g
So the solution would be prepared by dissolving 1.0 g of NaCl in 49.0 grams of water.
One 50-60 minute class period
Paper, pencil and calculator
Molarity is the number of Moles of solute divided by the number of liters or dm3 of solvent. The abbreviation for molarity is M .
Molality is the number of moles of solute divided by the number of kg of solvent. The abbreviation for molality is m .
% solution means the solvent is a given percentage of the total mass of the final solution.
Assume that all solutions are being dissolved in water. Assume that the density of water is 1.0 g/cm3 and that the final density of all solutions is 1.0 g/cm3. (Thus, 1 cm3 of solution equals 1 gram of solution.)
1. How would you make 5.00 X 102 cm3 of a 4.0 M NaCl solution? Dissolve 117 grams of NaCl in enough water to make 500.0 cm3 of solution
2. How would you make 1.00 X 103 cm3 of a 5.5 m KBr solution? Dissolve 655 grams of NaCl in enough water to make 500.0 grams of solution
3. How would you make 40.0 grams of a 5.0% KOH solution? Dissolve 2.0 grams of KOH in 38.0 grams of water.
4. How would you make 40.0 cm3 of a 2.0 % Mg(OH)2 solution? Dissolve 0.80 grams of Mg(OH)2 into 39.2 grams of water.
5. How would you make 50.0 cm3 of a 0.025 M NaF solution? Dissolve 52 grams of NaCl in enough water to make 50.0 cm3 of solution
6. How would you make 100.0 cm3 of a 0.50 m KCl solution? Dissolve 3.7 grams KCl in 100.0 grams of water.
7. How would you make 400.0 grams of a 7.0% Sr(OH)2 solution? Dissolve 28 grams of Sr(OH)2 into 372 grams of water.
8. How would you make 400.0 cm3 of a 4.0 % KNO3 solution? Dissolve 16 grams of KNO3 into 384 grams of water.
9. How would you make 4.0 grams of a 1.0% Na2SO4 solution? Dissolve 0.040 grams of Na2SO4 into 3.6 grams of water.
10. How would you make 70.0 dm3 of a 25 % Ba(NO3)2 solution? Dissolve 17,500 grams of Ba(NO3)2 into 52,500 grams of water.
11. How would you make 17.0 grams of a 2.0% sucrose solution? Dissolve 0.34 grams of sucrose into 16.7 grams of water.
12. How would you make 0.250 dm3 of a 2.50 % Ca(NO3)2 solution? Dissolve 6.25 grams of Ca(NO3)2 into 244 grams of water 13. How would you make 0.050 dm3 of a 1.0 % agarose? Dissolve 0.50 grams of agarose into 49.5 grams of water.
14. How would you make 0.050 dm3 of a 2.0 % agarose? Dissolve 1.0 grams of agarose into 49.0 grams of water.
15. How would you make 0.050 dm3 of a 3.0 % agarose? Dissolve 1.5 grams of agarose into 48.5 grams of water.
16. How would you make 0.050 dm3 of a 4.0 % agarose? Dissolve 2.0 grams of agarose into 50.0 grams of water.
17. How would you make 50.0 cm3 of a 1.0 % agarose? Dissolve 0.50 grams of agarose into 49.5 grams of water.
18. How would you make 50.0 cm3 of a 2.0 % agarose? Dissolve 1.0 grams of agarose into 49.0 grams of water.
19. How would you make 50.0 cm3 of a 3.0 % agarose? Dissolve 1.5 grams of agarose into 48.5 grams of water.
20. How would you make 50.0 cm3 of a 4.0 % agarose? Dissolve 2.0 grams of agarose into 50.0 grams of water.
The formula for an agarose monomer is C12H18O9
21. What is the molecular mass of agarose? 306.30 g/mole
22. Determine the molarity of a 1% agarose solution. (Hint: assume you are making 100 grams of total solution.) 0.0326 M
23. Determine the molarity of a 2% agarose solution. (Hint: assume you are making 100 grams of total solution.) 0.0652 M
24. Determine the molarity of a 3% agarose solution. (Hint: assume you are making 100 grams of total solution.) 0.0979 M
25. Determine the molarity of a 4% agarose solution. (Hint: assume you are making 100 grams of total solution.) 0.131 M