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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.

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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

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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

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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

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To find moles from mass: ◦ (Mass of Compound Given) x (1 mole/Molar Mass) = Number of Moles

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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

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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

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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

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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

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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

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Describe how to calculate the percent by mass of an element in a compound Interpret an empirical formula Distinguish between empirical and molecular formulas.

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How many people are in the room right now? How many are Male? Female? What percentage of the whole is Male? Female?

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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.

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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?

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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?

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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.

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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?

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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

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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?

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