Preparing Chemical Solutions

• A. Meaning of “Mass” and “Molecular Mass” [*]

• Ax. Exercises [*]

• B. Molecular Mass of a Few Molecules [*]

• Bx. Exercises [*]

• C. Meaning of “Mole”; Measuring Moles [*]

• Cx. Exercises [*]

• D. Meaning of “Concentration” [*]

• Dx. Exercises [*]

• E. Molecular Mass to Mass to Volume [*]

• Ex. Exercises [*]

• F. Recipe for a Given Concentration and Proportion [*]

• Fx. Exercises [*]

Determining the Current in a Circuit

• G. Reduction Potentials and Voltage, meaning of subtraction

• Gx. Exercises

• H. Resistivity and Resistance

• Hx. Exercises

• I. Current

• Ix. Exercises

Appendices

• Appendix A: The Periodic Table

• Appendix B: Table of Reduction Potentials

• Appendix C: Table of Resistivity

Standards:

• 2.2 Computation and Estimation

• 2.3 Measurement and Estimation

• 2.5 Mathematical Problem-Solving and Communication

• 2.8 Algebra and Functions

 

A. The Meaning of “Mass” and “Molecular Mass”

Anything with weight will weigh more on the earth than on the moon, and more in a valley than on a mountain. The weight of an object depends on gravity. As you raise an object above the earth, its weight is reduced. An object in outer space, far from all planets, may have no weight.

 

Mass does not depend on gravity. The mass of an object is the amount of matter it contains. If you weigh twice as much as your little brother, it is because you have twice his mass. Your weight will be different on the moon, but even on the moon, you will weigh twice as much as your brother, because you have twice as much mass.

Most Americans use a measurement system called the English system of measurement. It uses feet for length and gallons for volume. In the English system, units of weight (like pounds) must be used for mass too, because there are no different units of weight and mass.

 

But scientists in America, and most people in other countries, use the metric system instead. In the metric system, you use meters for length and liters for volume. And in the metric system, there are different units of weight and mass. Weight can be described in Newtons, and mass can be described in grams. Of course, even if we want to know about mass, we usually measure weight first, and then figure out mass by some assumption about the gravitational attraction.

 

Ax. Exercises

1. On Earth, I weigh 150 lbs., and my dog Ramachandran weighs 50 lbs. On the moon, I weigh 25 lbs.; what would Ramachandran weigh?

2. Etc.

 

Answers:

1. One correct approach is (Me on Moon / Me on Earth) = (Ramachandran on Moon / Ramachandran on Earth);

          (25/150) = (x/50);

          6x=50;

          x=50/6 

B. Molecular Mass of a Few Molecules

The formula for a molecule lists the number of atoms of each element in that molecule. For instance, water is identified by the formula H₂O; the formula H₂O indicates that there are 2 hydrogen atoms and 1 oxygen atom in each molecule of water. You find the mass of a molecule by adding the masses of the atoms that compose it.

Chemicals are delivered to labs in bottles that list the molecular mass, the mass of one molecule of the chemical inside. This is the same mass that you could find by the above method; it is listed on the label so that the scientists do not need to compute it themselves. The chemical may be in the form of a power; or, it may be in solution (dissolved in some other liquid). Scientists working with the chemical cannot of course count out a certain number of molecules: they must measure powders by weight, and liquids by volume.

 

You can find the mass of any number of molecules by multiplying that number by the mass of one molecule.

 

Bx. Exercises

1. What is the mass of one molecule of water?

2. What's the mass of two molecules of Calcium Chloride, CaCl3?

3. Etc.

Answers:

1. A periodic table of the elements lists the atomic mass of each element. On the table, you can see that each atom of hydrogen has a mass of 1.00794, and each atom of oxygen has a mass of 15.9994. (What are the units in these measurements?) Therefore, the mass of each water molecule is... 

 

  

C. Meaning of “Mole”; Measuring Moles

 A mole of a substance is a certain number of molecules of that substance, around

 

That number, six hundred sextillion, is also called 6*10²³. For some reason, this has become a standard "amount" of substance, a standard number of molecules. Scientists working in a lab may want a certain number of moles in each liter of solution. Often they want a total molarity (number of moles per liter), containing a given proportion of each molecule involved. So they must do some kind of math to figure out the weight (of a powder) or volume (of a solution) that contains the number of moles that they want.

 

Cx. Exercises

Find the mass of...

1. .5 moles of table salt (NaCl)

2. .75 moles of sugar (C₆H₁₂O₆)

3. Etc.

Find the number of moles composing...

1. .35 mg. salt (NaCl)

2. .60 mg. sugar (C₆H₁₂O₆)

3. Etc.

D. Meaning of “Concentration”

 

Suppose we take a drop of paint and mix it in water. The more water involved, the less color it will have. That is because as a fixed amount of paint spreads through larger amounts of water, the amount of paint in each cup of the mixture is lowered.

 

That concept, the amount of a substance in each volume of a mixture, is known as concentration. A more concentrated mixture of paint will look darker in color; a less concentrated mixture will look clearer.

 

If a drop of red paint is put into one gallon of water, each cup contains 1/16 of a drop of paint. If a drop of blue paint is put into two gallons of water, each cup of mixture contains 1/32 of a drop of paint. So the mixture of red paint is half as concentrated as the mixture of blue paint.

 

Of course, any amount of the blue mixture will have the same concentration. If you took one-third of the mixture, it would contain one-third the paint and one-third of the water, so be just as concentrated as the two-thirds you left behind. Concentration is stated as an amount per liter, not as an absolute amount of paint in the volume you happen to have taken.

 

Dx. Exercises

How many moles in each liter of water if you dilute

1. 5 mg. of table salt (NaCl) in 2 liters of water

2. 1.6 mg. of sugar () in 3 liters of water

3. Etc.

 

E. Molecular Mass to Mass to Volume

As you know, a chemical may be bottled as a solution. In that case, the label will explain the concentration, the mass of chemical in each volume of solution. Sometimes an experiment will require a molarity, a certain number of moles in each liter of mixture.

 

Ex. Exercises

Q: How many mL of 100 mg/mL solution would supply the correct amount of PC?

A: 5.76 mg PC * (mL/100 mg) =.576 mL 

 

Q: How many mL of (.0529 g/mL) Chol would supply the correct amount of Chol?

 

A: .96675 mg Chol * (mL/.0529 mg) =18.275047259 mL