The Mole Notes. There are many ways to or measure things. In Chemistry we also have special ways to count and measure things, one of which is the.

Transcription

1 The Mole Notes I. Introduction There are many ways to or measure things. In Chemistry we also have special ways to count and measure things, one of which is the. A. The Mole (mol) Recall that atoms of different elements have different. Since atoms are so small, we use the to represent the mass of an atom. The atomic mass is the of an expressed relative to the mass assigned to. Atomic mass is measured in which cannot be used in the lab. To measure the mass of things in lab, we need to use the. The is a way we can relate the mass of an atom in atomic mass units to the mass of an atom in. The mole is the of atoms of an element to the number of atoms in exactly 12.0 g of carbon-12. This number is X is called number after Amadeo Avogadro who did research to come up with this number. This number represents the number of,, or in one mole of a substance. To understand the size of this number, consider the following: 6.02 X sheets of paper stacked up would reach beyond our solar system 6.02 X basketballs could create a new planet the size of the earth 6.02 X grains of rice could cover all of the land on earth 75m deep 6.02 X pennies- if you had this much you could give away $1 million dollars a day for over 3000 years Therefore there are H atoms in 1.0 g of H atoms. There are O atoms in 16.0g of O atoms, etc.

2 We now know that the of 1 mole of any element is numerically equal to the of that element. The mole is like a (12 of something), a gross (144 of something). A mole represents of something (usually atoms). B. Molar Mass and Gram Formula Mass Using the mole we can now determine the mass of an element or compound in grams. 1. Molar Mass is the mass in grams of of a certain element. It is the same as the listed on the. Note: The numbers from the periodic table need to be rounded to the first decimal place before you record them. What is the molar mass of the following? C Mg S H Cu Hg I Br Au Ag Al Ca 2. Gram Formula Mass is the mass in grams of 1 mol of a like water. How would we determine this?

3 What is the gram formula mass of the following compounds? HC 2 H 3 O 2 CH 4 Cl 2 NaHCO 3 II. Mole Conversions Using the relationship between the mole, the number of grams and Avogadro s number, we can work conversions using the mole. 1 mol = 6.02 X particles, molecules, atoms or formula units 1 mol = molar mass (in g) from the periodic table A. Mole/Mass Conversions: How many moles are in 11.2 g of NaCl? How many grams are in 3.20 mol of Zn(NO 3 ) 2?

4 How many grams are in mol P 2 O 5? How many moles are in g of NaNO 3? B. Mole/Avogadro s Number Conversions How many molecules are in 2.0mol of water? 8.74 X formula units are equal to how many moles of CaCO 3? 0.36 mol of Al is equal to how many atoms of Al?

5 7.9 X formula units are equal to how many moles of Na 2 CO 3? C. Mole/Volume Conversions We also have a relationship between the and the at standard temperature and pressure (STP) (STP is a pressure of and a temperature of ) 1 mol of any gas at STP = 22.4 L How many moles are in 1.0 L of CO 2? 0.82 moles of O 2 is equal to how many liters of O 2?

6 D. Multiple Step Conversions Remember the three conversions: 1 mol = 6.02 X particles, molecules, atoms or formula units 1 mol = molar mass (in g) from the periodic table 1 mol of any gas at STP = 22.4 L 7.2 X10 22 atoms of Ca are equal to how many grams of Ca? 4.0 X molecules of CH 4 are equal to how many grams of CH 4? 1.8 g of NaHCO 3 is equal to how many formula units of NaHCO 3? 56.3 g of Cu is equal to how many atoms of Cu?

7 E. Percent Composition Problems Percent composition is the of each element in a compound compared to the of the compound multiplied by 100. Can be calculated in two ways: 1. Using a chemical formula 2. Using experimental data Calculate the percent composition of H 2 O. Calculate the percent composition of C 12 H 22 O 11. A g sample is made up of g C, g O and g H. What is the percent composition of the sample? A 5.00g sample is made up of 1.94g C, 0.48g H and 2.58g S. What is the percent composition of the sample?

8 F. Empirical and Molecular Formula Problems An empirical formula is a formula that represents the of atoms in a compound. Example: A compound is 80.0% C and 20.0% H by mass. What is the empirical formula for this compound? 1. To work this problem you first have to have the mass of each element. If the problem gives you percent by mass, assume you have 100g and the percent will be the same as the mass. For this problem that means we have 80.0g C and 20.0g H. 2. Convert the mass of each element to moles: 3. Find the ratio of each number of moles to the smallest number of moles. Example: What is the empirical formula for a compound with 13.5g Ca, 10.8g O and 0.675g H?

9 Example: What is the empirical formula for a compound made up of 40.0%C, 6.67%H and 53.3% O? After determining the empirical you can also determine the molecular formula. A Molecular formula will always be a of the empirical formula. For example, using the above problem, what would be the molecular formula for the above empirical formula if the gram formula mass for the molecular formula is g/mol?