Unit 6: Counting by Weighing

Counting by Weighing

This unit focuses on the idea that if you want to count out a very large number of items, the most effective way of doing this would be by weighing them and using some ideas about the item being weighed to figure out how many of those items you have.

This idea is applicable to chemistry, as the atoms and molecules that we work with are so so so small, that we cannot hope to actually count them. We need to be able to work with mass in order to determine how many of our atoms/molecules we have within a certain amount of a substance.

Intro Activity - Relative Mass

6a - Scientific Notation

Being able to work with scientific notation is important in science. In chemistry, we are often dealing with very very small numbers - like how much an individual proton weighs - or we are dealing with very very large numbers, like how many atoms or molecules there are in a weighable amount of a substance.

Scientific notation is just how we write very large and very small numbers without losing our minds counting up and writing out all the zeroes.

You should know how to:

  • Convert from scientific notation to standard form
  • Convert from standard form to scientific notation
  • Multiply any two numbers in written in scientific notation
  • Divide any two numbers written in scientific notation
  • Add any two numbers written in scientific notation
  • Subtract any two numbers written in scientific notation

6b - Working with Moles

Because atoms are so extremely small and light, scientists work with numbers of atoms or molecules using a very large number to count them.

This is just like how we count eggs by the dozen. If you needed a lot of eggs, like for an easter egg hunt, you wouldn't go to the store to pick out all those eggs, you would go to the store and just grab a few dozen eggs.

The version of the dozen that we use for counting atoms and molecules is called the mole. While a dozen is the same thing as 12, one mole is equal to 6.02 x 10^23. That's 602 with 21 zeroes after it. It's a very large number, because atoms are so small!

You should know how to:

  • Calculate the molar mass of a compound
  • Convert between two units, given a relationship between the two units
  • Convert from grams to moles of a compound
  • Convert from moles to grams of a compound