Counting by Weighing
By: Celene Hernandez
How do you order meat at the deli? By the slice? How do you buy fruit? Of course you purchase these items by weighing them! This unit, Sophomore IB Chemistry students at Roberto Clemente Community Academy learned about the concept of counting by weighing. Counting by weighing involves counting the number of particles in a sample by weighing them.
Students were shown a large mystery container of beans and asked to guess how many beans were in the container. Guesses ranged from 200- 3,000 beans! After students made their guesses and answered a few pre-lab questions, they were ready to begin the lab! Students took the mass of sample sizes of 10 and 20 particles of beans and rice. Once students did a few calculations they determined the “Average-of-Averages” mass of one particle of rice and beans. With this new information, students were then able to determine the weight of any given number of rice or beans. They calculated the weight of 100 particles, 1000 particles, and then applied their newfound knowledge to other instances where counting by weighing is necessary. Finally, students were given some information to determine the number of beans in the mystery container!
Students were given the weight of the container and the weight of the container with the beans. With this information and their “average-of-averages” mass of beans they calculated, students were then able to determine that the mystery container contained 4,114 beans, in which most students underestimated the total number of beans!
What did students think of this lab? Alexis Castillo said, “I thought that the counting by weighing lab was fast, quick, easy, and hands on which was really great!” As IB learners, students are continuously exposed to inquiry-based learning and are using hands on, real world applications to solve everyday problems.