# 10.2 Mole-Mass and Mole-Volume Relationships

10.2 Mole-Mass and MoleVolume Relationships 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. So Lets Review from Part 1 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 1023 Moles and Math 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 1023 particles)= The number of Moles Second, if we are looking for the number of particles: (Number of Moles Given) x (6.02 x 1023 particles/1 Moles)= The number of Particles Mole-Mass

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 Mole-Mass

To find moles from mass: (Mass of Compound Given) x (1 mole/Molar Mass) = Number of Moles Try the following Find the mass of : 4.5 x 10-3 moles of C20H42 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 N2O3 Mole-Volume Avogadros 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 Moles-Volume As we learned before, 1 mole of anything is equal to 6.02 x 1023 For gases, 1 mole is also equal to 22.4 L In other words a mole of ANY gas will occupy 22.4 L

Example If we have the following: 0.375 moles of O2 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 O2 = (0.375 mole) x (22.4 L /1 mole) Volume of O2 = 8.40 L Try the following

Calculate Volume of the given gases 3.2 x 10-3 mol of CO2 3.70 mol N2 1.25 mol He 0.335 mol C2H6 10.3 Percent Composition and Chemical Formulas Describe how to calculate the percent by mass of an element in a compound Interpret an empirical formula Distinguish between empirical and molecular formulas.

Lets Take a Step Back How many people are in the room right now? How many are Male? Female?

What percentage of the whole is Male? Female? Percent Composition 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. Lets Try One

Together.YAY! K2CrO7 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? Calculating Percent From Mass 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? Percent Comp from the Chemical Formula 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 CrO , let us 2 7 try and find the percent composition for

propane, C3H6. Percent Comp as a Conversion Factor When we looked at propane we could see a few things: The The The The

The molar mass is 44.0 grams mass of carbon was 36 grams mass of hydrogen was 8 grams Percent composition of carbon is 81.8% Percent composition of hydrogen is 18.2% But what would the masses be if we had 82 grams of propane?

Empirical vs Molecular Formulas An empirical formula is the lowest whole number ratio of the atoms in a compound. CH CH2O A molecular formula is either the same as the empirical formula, or it appears as a multiple from the empirical formula. C H 2 2

CH2O Finding the Molecular Formula of a Compound You are told the empirical formula of a substance is CH4N, and that you have 60 g/ mol of substance. What is the molecular formula?

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