**Purpose:** 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.

**Prior Knowledge:** Students need to be able to calculate a weighted average.

Avg. mass = Σ(mass of washer × relative abundance)

**Procedure:**- 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 | | | |

**Analysis: Part I**Calculate the average mass of the washers in the sample.

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!

**Analysis: Part II**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