Presentation on theme: " Objectives ◦ Describe how to convert the mass of a substance to the number of moles of a substance, and vice versa. ◦ Identify the volume of a quantity."— Presentation transcript:
Objectives ◦ Describe how to convert the mass of a substance to the number of moles of a substance, and vice versa. ◦ Identify the volume of a quantity of gas at STP.
A mole is a unit of measurement ◦ Kind of like a dozen = 12, or a ream = 500 A mole is a set number of particles, molecules, atoms, or just whatever it is we are talking about. ◦ That number is 6.02 x 10 23
If we wish to find the moles or particles of something we follow two easy set ups. ◦ First if we are looking for moles: (Number of Particles Given) x ( 1 mole/ 6.02 x 10 23 particles)= The number of Moles ◦ Second, if we are looking for the number of particles: (Number of Moles Given) x (6.02 x 10 23 particles/1 Moles)= The number of Particles
If we wish to convert between the moles and mass of a compound, we need the molar mass. ◦ For Example: If we wanted to see how many grams are present in 3 moles of NaCl, we simply do the following (Number of Moles) x (Molar Mass/ 1 mole) 3 moles of NaCl x (58.5 g/mole of NaCl) = 176 grams of NaCl
To find moles from mass: ◦ (Mass of Compound Given) x (1 mole/Molar Mass) = Number of Moles
Find the mass of : 4.5 x 10 -3 moles of C 20 H 42 2.5 moles of Fe(OH) 2 Calculate the Number of moles of: 3.7 x 10 -1 grams of Boron 75.0 grams of N 2 O 3
Avogadro’s Hypothesis states that equal volumes of gases at the same temperature and pressure contain equal number of particles. When we try and calculate using a gas, we need to confirm one item. That is STP, or standard temperature and pressure. At these perfect conditions we can progress in the calculations
As we learned before, 1 mole of anything is equal to 6.02 x 10 23 For gases, 1 mole is also equal to 22.4 L ◦ In other words a mole of ANY gas will occupy 22.4 L
If we have the following: 0.375 moles of O 2 gas, what volume will it occupy Volume of Gas = (Moles of Gas) x (22.4 L/1 mole) With this we get the following Volume of O 2 = (0.375 mole) x (22.4 L /1 mole) Volume of O 2 = 8.40 L
Calculate Volume of the given gases 3.2 x 10 -3 mol of CO 2 3.70 mol N 2 1.25 mol He 0.335 mol C 2 H 6
Describe how to calculate the percent by mass of an element in a compound Interpret an empirical formula Distinguish between empirical and molecular formulas.
How many people are in the room right now? How many are Male? Female? What percentage of the whole is Male? Female?
The percent by mass of an element in a compound is the number of grams of the compound multiplied by 100% Percent Composition is the percent by mass of each element in a compound.
K 2 CrO 7 How many Potassiums, Chromiums, and Oxygen do we have? Whats the total molar mass of the compound? What is the percent composition of the elements compared to the whole?
When a 13.60 gram sample of a compound containing magnesium and oxygen is decomposed,5.40 grams of oxygen is obtained. What is the percent composition of this compound?
We use the subscripts to tell us the amount of each element in a mole of compound, which in turn can tell us the mass, and percent composition. Similar to when we solved for K 2 CrO 7, let us try and find the percent composition for propane, C 3 H 6.
When we looked at propane we could see a few things: ◦ The molar mass is 44.0 grams ◦ The mass of carbon was 36 grams ◦ The mass of hydrogen was 8 grams ◦ The Percent composition of carbon is 81.8% ◦ The Percent composition of hydrogen is 18.2% But what would the masses be if we had 82 grams of propane?
An empirical formula is the lowest whole number ratio of the atoms in a compound. ◦ CH ◦ CH 2 O A molecular formula is either the same as the empirical formula, or it appears as a multiple from the empirical formula. C 2 H 2 CH 2 O
You are told the empirical formula of a substance is CH 4 N, and that you have 60 g/mol of substance. What is the molecular formula?