I II III IVcreated by C. Johannesson modified by T. Howard

Chemistry Unit 7: Moles

C8A- I will define & use the concept of a mole.

C8B – I will use the mole concept to calculate the number of atoms, ions, or molecules in a sample of material.

C8C – I will calculate percent composition & empirical and molecular formulas.

A. mole – amount of substance that has particles equal to the number of atoms in 12g of carbon 12

OR the amount of substance that has Avogadro’s # of particles ---

6.02 x 1023 particles

A large amount!!!!

1 mole of hockey pucks would equal the mass of the moon!

A. What is the Mole?

1 mole of pennies would cover the Earth 1/4 mile deep!

1 mole of basketballs would fill a bag the size of the earth!

B. Avogadro’s number – number of particles in 1 mole of a pure substance 6.02 x 1023

C. atomic mass number – number of protons + neutrons; measured in amu’s

---this is NOT on the p-table

D. average atomic mass number – average weighted mass of an atom/element

---this is on the periodic table

If you change the units from amu to grams you now have the amount of grams in 1 mole of the element – something you can actually measure on a scale!

avg atomic mass calculation

( % isotope #1) ( mass isotope #1) + ( % iso #2) ( mass iso #2) +…

100

C6D Use isotopic composition to calculate average atomic mass of an element.

The natural abundance for boron isotopes is: 19.9% 10B (10 amu) and 80.1% 11B (11amu). Calculate the atomic weight of boron.

[(19.9 × 10) + (80.1 × 11)]

100

avg atomic mass is 10.801 amu

E. Molar mass – mass of 1 mole of a pure substance----units are grams/mole or g/mol

Turn to p.7 in your packet (Molar Mass Wkst 3)! Look at right side of page & we will do a couple

of examples…the rest is for you to complete

1. Molar mass is “equal” to the mass # of an element in amu’s, so if an element has an mass # of 39.0983 amu’s then the molar mass is 39.0983 g/mol you just change the unit of measurement!

F. formula mass aka formula weight– sum of all the mass numbers of the atoms in a compound

the term formula mass is usually associated with ionic compounds &

the term molecular mass is usually associated with covalent compounds

they’re the same basic thing

#3 – BeCl2Be = 1 x 9.012 g = 9.012 g

Cl = 2 x 35.453 g = + 70.906 g

79.918 g

from the periodic table

formula mass of BeCl2 aka molar mass

number of atoms in the formula

#7 – Mg(OH)2

Mg = 1 x 24.305 = 24.305 g

O = 2 x 15.999 = 31.998 g

H = 2 x 1.008 = 2.016 g

f.m. of Mg(OH)2 = 58.319 g

You need to do the rest on your own!

G. percent composition- percent by mass of each element in a compound

mass of element x 100 = % element in cmpd

mass of total cmpd

What percent by mass of Mg(OH)2 is oxygen?

find the formula mass of the Mg(OH)2

Mg = 1 x 24.305 = 24.305 g

O = 2 x 15.999 = 31.998 g

H = 2 x 1.008 = 2.016 g

f.m. of Mg(OH)2 = 58.319 g

mass of element x 100 = % element in cmpd

mass of total cmpd

31.998g O x 100 = 54.87% oxygen by mass

58.319g Mg(OH)2

That means more than half of the mass of the Mg(OH)2 (what is the name of the compound?) is due to oxygen!

H. % error= (observed value - actual value) x 100

actual value

Observed value refers to the data you got in the lab while actual value is the “correct” or “book” value/answer.