Measure Masses of Common Compounds, Objects and Minerals

Weighing: A Means of Counting

Objectives:  Measure masses of common compounds, objects and minerals.  Calculate moles and atoms from experimental masses.

Introduction: You can often measure how much of something you have by counting individual objects. For example, you can count the number of pennies you have in your pocket or the number of pencils you have in your locker. You learned earlier in the year in chemistry that there is a name for the number of atoms, ions, or molecules. One mole of a substance is equal to 6.02 x 1023 atoms, ions, or molecules of that substance. You also learned that you can “count” the number of moles in a substance by weighing the substance.

Purpose: In this lab you will measure the masses of samples of various common compounds like water, salt, and sugar. You will use your measurements as a means of counting the atoms, ions, and molecules in your samples. You will extend your technique to common objects that you can consider to be pure substances, like glass marbles, pieces of chalk, and polystyrene peanuts. Finally you will measure the masses of various mineral samples and use your results to find the number of atoms in each.

Procedures: 1. Weigh on level teaspoon of sodium chloride. Record the mass of the sodium chloride. Repeat this with water and sucrose. 2. Weigh and record the mass of a glass marble, a piece of chalk, and a polystyrene peanut. 3. Weigh and record the mass of a piece of sulfur, a piece of fluorite, and a piece of hematite.

Results: 1. Calculate the gram formula mass for each of the substance for which you measured the mass. 2. Calculate the number of moles of each substance measured. 3. Calculate the number of moles of each element in the substances measured. 4. Calculate the number of atoms of each element in the substances measured.

Conclusions: 1. In step 1 of the procedures, you measured equal volumes of three different compounds. Which of the three compounds has the greatest number of moles in one teaspoon? 2. Which of the three compounds in step 1 has the greatest total number of atoms? 3. Why can we use this technique of measuring volume as a means counting? 4. Design an experiment that will determine the number of atoms of calcium, carbon, and oxygen it takes to write your name on a sidewalk with a piece of chalk. Assume the chalk is

100 percent calcium carbonate, CaCO3. 5. A nickel coin is a mixture of metals called an alloy. It consists of 75% copper and 25% nickel. Design and experiment to find out how many nickel atoms there are in one 5-cent piece. Above and Beyond:

 A common mineral used in wallboard and plaster of Paris is gypsum, CaSO4·2H2O. Gypsum is an example of a hydrate. A hydrate is a compound that has water molecules incorporated into its crystalline structure. The chemical formula for gypsum indicates that there are two water molecules for every calcium and sulfate ion within the crystal structure of gypsum. These water molecules are called water of hydration. Your next experiment will involve using a hydrate. If you hang a piece of sheetrock in your house that is 4.0 ft x 8.0 ft x 0.50 inches, how many molecules of gypsum are in that piece of sheetrock? The sheet rock has a density of 2.35 g/cm3.