Counting by Weighing

Who wants to count 100 M&M’s?

0Suppose you work at a candy store…would you want to count out M&M’s one by one?

0 If you think about it, it makes way more sense to use a scale and count the M&M’s by weighing them

0What would you need to know?

Average Mass

0Obtain the mass of 5 different M&M’s.

0How do we determine the average mass?

0For counting purposes we assume all the items behave as though they were identical.

Now what if we have 2 kinds of candy?

0 I want one bag of gum drops and one bag of M&M’s but they both need to have EXACTLY the same number of items.

0How would I figure this out?

MAIN IDEA!!!

0 Items can have different masses yet represent the same number of items.

Atomic MassesCounting by Weighing

Keep in mind the gum drop/M&M example

0Atoms are extremely TINY so normal units like grams and kilograms are way to LARGE.

0For example, the mass of a single carbon atom is 1.66 x 10-24 grams

Atomic Mass Units (amu)

0To avoid using exponents like 10-24 , scientists defined a much smaller unit of mass called atomic mass units (amu)

01 amu = 1.66 x 10-24 grams

Using Atomic Mass Units

0Let’s think about

0Average Atomic Mass = 12.01 amu

0What mass of carbon atoms must we have to have 1000 carbon atoms present?

Using Atomic Mass Units Continued…

0We weigh a pile of carbon atoms and the result is 3.00 x 1020 amu. How many carbon atoms are present?

0Recall 1 carbon atom = 12.01 amu

Using Atomic Mass Units Continued…

0These principles and calculations apply to all of the other atoms

0Atomic mass on the PTE refers to amu0Do the following:

1. What is the mass in amu of a sample containing 75 aluminum atoms?

2. Calculate the number of sodium atoms present in 1172.49 amu.

THE MOLE!!!!!

0AMU’s are extremely small units

0 In lab, we commonly use grams. How do we count atoms in samples with masses given in grams?

Visual representations

0 If we weigh out samples of all the elements such that each sample has a mass equal to that element’s average atomic mass in grams, these samples all contain the same number of atoms

The Mole

0This number (the number of atoms presents in all the samples) is called the mole.

0Mole = the number equal to the number of carbon atoms in 12.01 grams of carbon

0This number has been determined to be 6.022 x 1023 (Avogadro’s number)

The Mole

0One mole of something always contains 6.022 x 1023 units of that substance.

0Think about the concept of 1 dozen

0A sample of an element with a mass equal to that element’s average atomic mass expressed in grams represents 1 mol of atoms

12.011 grams of Carbon= 1 mole of carbon

Using the mole in calculations

0A sample of hydrogen weighs 0.500 grams. How many moles of hydrogen are present?

0What is the mass of 1 mole of hydrogen?0 1 mole of hydrogen = 1.008 g

Calculations Continued…

0We know the mass of 1 mol of H atoms so we can determine the number of moles of H atoms in any other sample by comparing its mass with the with the mass of 1 mole of H atoms.

0We can follow this process for any element

Calculations Continued…

0Once we know how many moles of something we have, we can figure out how many individual units are present

01 mole = ? Units01 mole = 6.022 x 1023 units0Recall our example…0.496 moles of H. How many

atoms of H are present?0DIMENSIONAL ANALYSIS

Now you give it a try

0Compute the number of moles and the number of atoms present in 10.0g of aluminum.

A more complicated example…

0How many silicon atoms are present in a 5.68 mg sample of silicon.