To understand the concept of average mass To learn how counting can be done by weighing

1. Objectives • To understand the concept of average mass • To learn how counting can be done by weighing • To understand atomic mass and learn how it is determined • To understand the mole concept and Avogadro’s number • To learn to convert among moles, mass, and number of atoms

2. Some Questions……… • How many atoms are there in a bucket of water? How could we determine that without having to count them all individually? • I know that one atom of carbon reacts with one molecule of oxygen to make one molecule of carbon dioxide. If I have a sample of carbon how can I predict how much oxygen to react with it? Can I do it by counting out atoms? C + O2 reacts to give CO2 • What do the atomic masses in the Periodic Table stand for? periodic table

3. A. Counting by Weighing • We can count objects by knowing how much each one weighs and weighing the sample • Objects do not need to have identical masses to be counted by weighing. • All we need to know is the average mass of the objects. • To count the atoms in a sample of a given element by weighing we must know the mass of the sample and the average mass for that element. =

4. A. Counting by Weighing Averaging the Mass of Similar Objects • Example: How can we count out 1000 jelly beans? • Not all jelly beans have the same mass. • Suppose we weigh 10 jelly beans and find: • Now we can find the average mass of a bean. • Finally we can multiply to find the mass of 1000 beans. We can then weigh out 1000 beans.

5. B. Atomic Masses: How much do Atoms Weigh? • Atoms have very tiny masses so scientists made a unit to avoid using very small numbers. • 1 atomic mass unit (amu) is 1/12 of the mass of a 12C atom • 1 amu = 1.66 10-24 g How many amu’s does a 12C atom weigh? How many grams does a 12C atom weigh?

6. B. Atomic Masses: Counting Atoms by Weighing • The average atomic mass for an element is the weighted average of the masses of all the isotopes of an element. • The Periodic Table shows the atomic mass of each element in amu’s. This is different to the mass number periodic table What is the average atomic mass in amu’s of an atom of: Barium Fluorine Xenon Nickel

7. Calculate the Average Atomic Mass of Naturally Occurring Iron: Sum of the final column = 55.845 amu

8. Calculate the atomic masses of the following elements Use the isotope masses and %abundance from the Dynamic Periodic Table – Cu, C, K

9. The MOLE!!?!?! • A spicy sauce flavored with chocolate, usually served with turkey or chicken • Any of various small insectivorous mammals, esp. of the family Talpidae, living chiefly underground, and having velvety fur, very small eyes, and strong forefeet. • A spy who becomes part of and works from within the ranks of an enemy governmental staff or intelligence agency. • A large, powerful machine for boring through earth or rock, used in the construction of tunnels • A small, congenital spot or blemish on the human skin, usually of a dark color, slightly elevated • A very large number: 6.022 x 1023 • The atomic or molecular weight of a substance expressed in grams

10. C. The Mole Multiple Units • One dozen eggs is twelve eggs • One class is twenty students • One kilometer is 1000 meters • One alphabet is …………? • One Liter is ………..? • One gross is………...? Can you think of some multiple units?

11. C. The Mole • One mole of anything contains 6.022 x 1023 units of that substance. • Avogadro’s number is 6.022 x 1023. • A sample of an element with a mass equal to that element’s average atomic mass (expressed in g) contains one mole of atoms.

12. C. The Mole

13. One Mole of marbles would cover the USA to a depth of approximately 60 km!!! One marble has a diameter of 1cm and covers approx. one sq cm Area of USA is 10 million sq km = 107 sq km = 1017 sq cm Depth of marbles in a mole is 6 x 1023 / 1017 = 6 x 106 cm = 60 km

14. If you had a ball of string that was 1 mole of millimeters long would it reach around the Earth? (The circumference of the Earth is 40,076 km)

15. How many grams do the following weigh? 1.00 mol Se 2.00 mol Fe 72.5 mol Pb 0.102 mol Mg 0.521 mol Ni 1.23 x 10-3 mol Pt

16. How many moles in the following samples? 26.2 g Gold 41.5 g Calcium 335 mg Barium 12.01 g Carbon 1.42 x 10-3 g Palladium (P211 Q16)

17. Summary • One amu is one-twelfth of the mass of a 12C atom • One amu is close to the mass of one proton or one neutron. • One amu is a very small mass – 1.66 x 10-24 g • One mole is 6.022 x 1023 units of anything • One mole (of atoms) of an element will have a mass in grams equal to the mass in amu of one atom of that element • “One Mole” refers to 6.022 x 1023 atoms or to the atomic mass expressed in grams of an element

18. Some Questions……… • How many atoms are there in a bucket of water? How could we determine that without having to count them all individually? • I know that one atom of carbon reacts with one molecule of oxygen to make one molecule of carbon dioxide. How many grams of oxygen react with 12 grams of carbon to make carbon dioxide? C + O2 reacts to give CO2 • How are the atomic masses in the Periodic Table different to mass numbers?