PACKET #6 Moles/Stoichiometry Reference Table: T & PT

Gram Formula Mass (GFM) The GFM is the total mass of any substance (element or compound). Also referred to as molar mass or molecular mass. Ex: K2CO3 (2 K atoms, 1 C atom, & 3 O atoms) Mass K = 39g Mass C = 12g Mass O = 16g (39x2) + (12x1) + (16x3) = 138 grams The gram formula mass of K2CO3 = 138g

Molecular Formula (MF): indicates the total number of atoms of each element needed to form the molecule (actual ratio of atoms). Example: C4H10 Empirical Formula (EF): the simplest ratio in which atoms combine to form a compound. Example: C4H10 can be divided by 2 to give you C2H5 Molecular Mass: the total mass of the molecular formula Empirical Mass: the total mass of the empirical formula

Calculating Molecular Formula When supplied with the Empirical Formula, and the Molecular Mass, we are capable of calculating the Molecular Formula with the following equation: Mass MF Mass EF Example: The empirical formula is CH and the molecular mass is 26, what is the molecular formula?

Mole (mol): The SI base unit that measures an amount of a substance; 1 mole of molecules has a mass equal to the molecular weight in grams. 1 mole = 6.02x1023 atoms (molecules). When looking at a chemical reaction, the number of moles are indicated by the coefficients in front of the formula’s. Avogadro’s Number: The number of molecules in one mole for any substance; 1mole of particles = 6.02 x 1023 2 moles of particles = 1.204 x 1024 3 moles of particles = 1.806 x 1024 etc . . .

WHAT YOU KNOW!!!! SOOOOOO IMPORTANT!!! GFM = 1 mole = 6.02 x 1023 molecules = 22.4 L This is the most important information for you to memorize in order to master this unit. REMEMBER DIMENSIONAL ANALYSIS??? If you know the equivalent values above, then you can easily convert one value to another value. Example: How many grams in 4.5 moles of CO2?

WHAT YOU KNOW!!!! GFM of K2CO3 = 138 grams 138 g K2CO3 = 1 mole 1 mole K2CO3 = 6.02 x 1023 atoms of K2CO3 6.02 x 1023 atoms of K2CO3 = 22.4 L K2CO3 With “what you know”, you can set up a proportion to solve for any problem.

Converting (ANYTHING) Example: How many molecules are there if you have 4.2 moles of NaCl? Start with what you know!! Set up a dimensional analysis based on what you know, and what you are given.

Converting (ANYTHING) Example: How many liters are there in 3.8 moles of MgF2? Example: How many molecules in 104.2 L of ZnO?

MoleMass / MassMole Converstions Table T: mole = given mass GFM Example: If you have 54 grams of LiF, how many moles do you have? Example: If you have 2.3 moles of CuCl2, how many grams do you have?

% composition = mass part x 100 Percent Composition The percentage by mass of each of the elements in compound Table T: % composition = mass part x 100 mass whole Example: What is the percent by mass of magnesium in magnesium oxide (MgO)?

Percent Composition Hydrate: a compound that incorporates water molecules into its fundamental solid structure. The compound has a dot after it followed by the number of water molecules attached. EX:CaSO4•2H2O The dot in the hydrate equates to an addition sign when calculating the GFM of a hydrate. Anhydrate: Hydrate minus H2O. The water is evaporated from the original hydrate.

To calculate the percent of water in a hydrate, you must determine the mass of the whole, and the mass of the part. Example: Determine the percent composition of water in CaSO4•2H2O. GFM CaSO4•2H2O = whole GFM 2H2O = part

In lab, we will be performing an experiment called “Water in a Hydrate” in which we will take a hydrate and remove as much water from a given amount by heating it. The purpose of removing the water is to turn the hydrate into what is called an anhydrous, which is a hydrated compound minus the bonded water molecules. By calculating the amount of water in the sample of hydrate we can determine the “part” in the equation, and therefore calculate the percent composition of water in the hydrate.

Stoichiometry: The quantitative relationships between the amounts of reactants and products formed during a chemical reaction. 2H2 + O2  2H2O Notice the ratio between all of the substances in this reaction. Stoichiometry implies that no matter how you change the quantity of any of these substances, the others will have to change in order for the reaction to remain the same.

Stoichiometry Ca + 2H2O  Ca(OH)2 + H2 Mole-Mole Problems: Answers how many moles of one element or compounds react with a given number of moles of another element or compound. Example: How many moles of Ca are needed to react completely with 6 moles of H2O in the following reaction: Ca + 2H2O  Ca(OH)2 + H2 Cross out anything in the equation that the problem is not considering. Write the coefficient of each substance remaining under the element or compound. On top of the element or compound write the number of moles given in the problem. Set up a proportion:

Mass-Mass Problems: Answers how many grams of one element or compounds react with a given number of grams of another element or compound. Example: How many grams of calcium are needed to react completely with 108 grams of H2O in the following equation: Ca + 2H2O  Ca(OH)2 + H2

Review Questions 1) What is the total number of moles of hydrogen gas contained in 9.03 x 1023 molecules? A) 6.02 moles B) 2.00 moles C) 9.03 moles D) 1.50 moles 2) What is the gram formula mass of Na2CO3• 10H2O? A) 266 g B) 286 g C) 100 g D) 142 g

3) The total number of molecules in 34.0 grams of NH3 is equal to A) 1.00 x 6.02 x 1023 B) 1.00 x 22.4 C) 2.00 x 22.4 D) 2.00 x 6.02 x 1023 4) An 8.24-gram sample of a hydrated salt is heated until it has a constant mass of 6.20 grams. What was the percent by mass of water contained in the original sample? A) 75.2% B) 14.1% C) 24.8% D) 32.9% 5) A compound with an empirical formula of CH2 has a molecular mass of 70. What is the molecular formula? A) C2H4 B) CH2 C) C4H8 D) C5H10

6) What is the percent by mass of oxygen in magnesium oxide, MgO? A) 50% B) 60% C) 40% D) 20% 7) Given the reaction: PbCl2(aq) + Na2CrO4(aq)‚ PbCrO4(s) + 2NaCl(aq) What is the total number of moles of NaCl formed when 2 moles of Na2CrO4 react completely? A) 1 mole B) 2 moles C) 3 moles D) 4 moles 8) What is the percent composition by mass of aluminum in Al2(SO4)3 (gram-formula mass = 342 grams/mole)? A) 15.8% B) 7.89% C) 36.0% D) 20.8%

9) What is the mass in grams of 3.0 x 1023 molecules of CO2? A) 66 g B) 88 g C) 44 g D) 22 g 10) Which compound has the empirical formula CH? A) C3H8 B) CH4 C) C6H6 D) C2H4 11) The gram formula mass of NH4Cl is A) 22.4 g/mole B) 28.0 g/mole C) 95.5 g/mole D) 53.5 g/mole

12. In the lab, a student performed an experiment to determine the percent by mass of water in a hydrate. The following data was recorded: Mass of crucible + cover = 11.70g Mass of crucible + cover + hydrated salt before heating = 14.90g anhydrous salt after heating = 14.53g Determine the approximate percent by mass of water in the hydrated salt (show all work).

AIM: How do we calculate the percent composition by mass of water in a hydrate? HW: Complete WS on Hydrates