6.02 x 1023
atoms!
Chemistry Ch. 10 Lab: Who is counting?
Objective: To count the number of particles in a sample by weighing.
points
possible
Heading (name, date, period)
Pre-lab questions
Data tables
Post-lab questions
Lab technique (group work together, follows directions)
TOTAL (15)
student
assessment
points
earned
1
3
2
6
3
15
Pre lab 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:
Beans, dried, about 75 g
Weighing dishes, 4
Whole rice, about 10 g
Electronic balance
Procedure:
Part A
1. Measure and record the mass of a weighing dish.
2. Count out the appropriate number 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 beans instead of rice.
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Data Table A. Average Mass of Rice and Beans
Sample
Mass
Weighing Dish
Weighing Dish and Sample
“10”
Sample
Average mass of one “particle”
“20”
Rice
Beans
Weighing Dish
Weighing Dish and Sample
Sample
Average mass of one “particle”
“Average-of-Averages”
Mass of one particle
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 beans instead of rice.
Data Table B. Counting by Weighing
Mass
Predicted mass of 100 particles
Weighing dish
A
Weighing dish and sample
Sample
Number of Particles
Weighing dish
Weighing dish and sample
B
Sample
Number of Particles
Rice
Beans
POST LAB 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,
(average _ number _ of _ particles )  100
% _ error 
*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 beans was found to be 143.85 g. The rice
grains were separated from the beans by putting the mixture through a large strainer (the
small rice grains fell through the holes, the larger beans 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 beans in the mixture. Then express the ratio of rice grains to beans 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 1023 (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 beans.
(1 pt) mass of rice grain x 602 = food molar mass of rice
5. (1 pt) What is a food molar mass in this lab and where is it leading for Chapter 10:
Chemical Quantities? What is a mole? Why do we use moles?
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