6.02 x 10 23 atoms!
Chemistry Activity: Counting by Weighing
Objective: To count the number of particles in a sample by weighing.
Heading (name, date, period, lab partner) points possible
1 student assessment points earned
Introductory questions
Post-activity questions
3
2
6
Data tables
Group technique (group work together, follows directions) 3
TOTAL (15) 15
Introductory questions:
“Counting by weighing” is used in other applications besides chemistry. A hardware store may sell nails in packages of 500 for example. Similarly, an office supply store may carry boxes that contain 100 paper clips. Do you think someone counts out by hand every nail or paper clip in these products?
1) The average mass of one paper clip is 0.39 g. What is the expected mass of 100 paper clips? (Show work)
(1 pt)
2) A paper clip manufacturer finds it is more efficient to package paper clips in 100-gram lots. How many paper clips would be contained in a 100-g package?
(1 pt)
3) In designing a label for this package of paper clips, how many paper clips would you recommend the label advertise? Why?
(1 pt)
Materials:
Blackeyed Peas (or Navy beans), dried, about 75 g Whole rice, about 10 g
Weighing dishes, 4 Electronic balance
Procedure:
Part A
1.
Measure and record the mass of a weighing dish.
2.
Count out the appropriate number (10 or 20) of individual rice grains into each weighing dish.
3.
Measure and record the combined mass of each weighing dish and rice sample.
4.
Calculate the average mass of one rice grain in each sample “10” and “20”.
5.
Calculate the “average of averages” to determine the average mass of a single grain of rice.
6.
Repeat steps 3-7 using blackeyed peas or beans instead of rice.
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Data Table A. Average Mass of Rice and Peas/Beans
Sample
“10 particles”
“20 particles”
Weighing Dish
Mass
Weighing Dish and Sample
Sample
Average mass of one “particle”
Weighing Dish
Weighing Dish and Sample
Sample
Average mass of one “particle”
“Average-of-Averages”
Mass of one particle
Rice Peas or Beans
PART B
1.
Label two dishes “A” and “B” and measure and record the mass of each weighing dish.
2.
Use the average mass of a single grain of rice to calculate the predicted mass of 100 rice grains.
3.
Measure out two separate samples, each with this predicted mass of rice grains into weighing dishes A and B respectively. Note: It may not be possible to obtain the exact predicted mass. Get as close as possible-whether above or below the predicted value.
Remember to take into account the mass of the weighing dish (zero your balance).
4.
Count the actual number of rice grains in each sample A and B.
5.
Repeat steps 1-4 using peas or beans instead of rice.
Data Table B. Counting by Weighing
Mass
Predicted mass of 100 particles
Rice Peas or Beans
Weighing dish
A
B
Weighing dish and sample
Sample
Number of Particles
Weighing dish
Weighing dish and sample
Sample
Number of Particles
Post-Activity Questions (write answers in complete sentences):
1.
In part A, does the average mass depend on the number of particles in the sample? What are the advantages and disadvantages of using the “average of averages” mass to calculate the expected mass of 100 particles?
(1 pt)
2. Find the average number of particles in samples A and B for both rice and beans.
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Then, using the equation below, calculate the percent error in the “counting by weighing” method for rice and beans,
_ number _ particles )

100
% _ error

( average _ of
100
* 100
1) Is this method more accurate for rice or beans?
2) Give a possible explanation for any difference in the accuracy of the method for rice and beans.
(1 pt) average # of particles of rice:
% error for rice: average # of particles of beans:
% error for beans:
Accuracy? Explanation?
3. The mass of a mixture of containing both rice and blackeyed peas was found to be 143.85 g. The rice grains were separated from the blackeyed peas by putting the mixture through a large strainer (the small rice grains fell through the holes, the larger blackeyed peas did not). The mass of the rice that separated out was 4.65 g. Use the results of the above experiment to estimate the number of rice grains and blackeyed peas in the mixture. Then express the ratio of rice grains to blackeyed peas in this mixture to the nearest whole number (e.g. 1:2, 2:1, 1:3, etc). Show work.
(2 pt) (mass of rice mixture = mass of grain of rice x # of rice particles) divide # of particles by smaller number to get a 1: x ratio and then estimate
4. The mass of a single grain of rice is extremely large compared to the mass of a single atom. (A typical hydrogen atom has a mass of 1.66 x 10
-24
g – too small to even imagine!).
Chemists, therefore, count atoms in large groups, called moles, where one mole contains
6.02 x 10
23
(Avogadro’s number) of atoms. Let’s define a food mole as containing 602 particles (rice, beans, peas etc.) Calculate the mass of one food mole of rice. This is the food molar mass of rice. Then, calculate the food molar mass of blackeyed peas.
(2 pt) mass of rice grain x 602 = food molar mass of rice
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