This activity will illustrate how to find the average weight
of a item in a sample without having to weigh every object in the sample. The average mass
of a washer in a sample will be determined by finding the mass
of just one representative item of each type and determining the % of that item within the population. Time required:
1 day. Group Size:
Students should work in groups of 2 Materials Needed Per group:
- A set of washers in a film canister. The set will contain washers of three different sizes. The composition of the canisters will vary from group to group.
Students need to be able to calculate a weighted average.
Avg. mass = Σ(mass of washer × relative abundance)Procedure:
Analysis: Part I
- Obtain a film canister with washers.
- Find the mass of one large washer, one medium sized washer, and one small washer. Record the masses.
- Determine the percentage of each type of washer. Record the percentages.
| ||Small washer||Medium washer||Large washer|
|Mass of ONE washer|| || || |
|% of washers|| || || |
Calculate the average mass of the washers in the sample.Analysis: Part II
Average mass = (mass of a small washer x percentage of small washers) + (mass of a medium washer x percentage of medium washers) + (mass of a large water x percentage of large washers)
NOTE: The percentages must be converted to decimal format!
Find the average mass of a washer by weighing all of the washers in the sample and dividing by the total number of washers.
|Mass of all washers||Total # of washers|
| || |
Average mass = mass of all washers/total # of washers × 100
Using the result from part one as the experimental value and the result from part two as the theoretical value, calculate the % error.
% error = (T-E)/T × 100