# It’s a beauty mark… It’s a small furry garden pest… No, wait… its how we count ATOMS!

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It’s a beauty mark… It’s a small furry garden pest… No, wait… its how we count ATOMS!

The mass of 1 mole of a substance.

Calculate the molar mass of 1 mole of magnesium chloride. (first you need a formula) Calculate the molar mass

FORMULAS - review MgCl 2 The subscript is the number at the bottom of a formula. There is 1- Mg & 2 - Cl

How to calculate molar mass 1.Identify the # of atoms of each element 2.Multiply # atoms by the atomic mass of that element. (round to 2 #’s after the decimal) 3.Add them all together 4.Grams (g) is the unit

Molar Mass MgCl 2 Mg – 1 (24.31) = 24.31 Cl – 2 (35.45) = 70.90 or 95.21 g/mol 95.21 g MgCl 2 1 mole MgCl 2 =

Molar mass allows scientist a way to measure moles in a lab. Molar mass of Fe = Molar mass of O 2 = Molar mass of Cu(OH) 2 = Molar Mass

Calculate the molar mass: (you MUST write the formula correctly before answering) 1 mole of Ammonium phosphide 1 mole of Trinitrogen pentachloride

The percent BY MASS of each element in a compound – divide the element’s total mass (part) by the molar mass (whole) then multiple by 100 to get the percent. Ex: % composition of MgCl 2 Mg – 1 (24.31) = 24.31 / 95.21 x 100 = 25.53% Mg Cl – 2 (35.45) = 70.90 / 95.21 x 100 = 74.47% Cl Molar mass = 95.21 g/mol Percent Composition (PART) (WHOLE) (PART)(WHOLE)

K – 1 (39.10) = 39.10 / 158.04 x 100 = 24.74% K Mn – 1 (54.94) = 54.94 / 158.04 x 100 = 34.76% Mn O – 4 (16.00) = 64.00 / 158.04 x 100 = 40.50% O molar mass KMnO 4 = 158.04 Practice - Calculate the % comp of KMnO 4 :

1.Find the % comp of the element in the compound 2.Change the % to a decimal (move decimal 2 times to the left or divide by 100) 3.Multiply that decimal by the amount (g) of the sample. Ex: Calculate amount of chlorine in 203.5 grams of MgCl 2. (use the % we found earlier) 74.47% Cl =.7447 x 203.5 = 151.5 grams Cl Calculating the amount of an element in a sample

H – 2(1.01) = 2.02 O – 1(16.00) = 16.00 Molar mass H 2 O = 18.02 Practice: Calculate the amount of oxygen in 15.75 grams of water. / 18.02 x100 = 88.79% O 88.79% O =.8879 x 15.75 = 13.98 grams O Complete % comp worksheet

The lowest whole number ratio (subscripts) of elements in a compound. Cannot be reduced!!! not empirical empirical Ex: C 6 H 12 O 6  CH 2 O Empirical Formula

Actual number of atoms in a chemical compound molecular EX: C 12 H 24 O 12 Molecular Formulas can be reduced to Empirical Formulas molecular empirical EX: C 12 H 24 O 12  CH 2 O Different molecular formulas can have similar empirical formulas molecular empirical EX: N 3 O 9  N 12 O 36  Molecular Formula Molecular formula: C 76 H 52 O 46 Empirical formula: ___________ NO 3

1.C 2 H 4 2.NO 3 3.S 9 Cl 12 4.C 3 Cl 9 5.N 4 S 9 PRACTICE: 1. Identify each as empirical (can’t be reduced) or molecular (can be reduced) 2. If its molecular – write the empirical molecular empirical CH 2 - empirical S 3 Cl 4 - empirical CCl 3 - empirical

Ex: A compound was found to be 54.53% Carbon, 9.15% Hydrogen, and 36.32% Oxygen. Find its Empirical Formula. Steps: Assume a 100g sample (change %  g) Use molar mass to find moles of each Divide all moles by the smallest number of moles Round each to the nearest whole # (sometimes you have to multiply to get a whole number - special) The resulting whole #are the subscripts for that element in the empirical formula Finding Empirical Formula from Percent Composition

63.5% Silver 8.2% Nitrogen 28.3% Oxygen 63.5 g Ag8.2 g N28.3 g O 107.87 14.01 16.00.589 mole Ag.59 mole N1.77 mole O.589.589.589 1 13 AgNO 3 Calculating Empirical Formula

60.00%C4.48%H 35.53%O 60.00g C4.48g H35.53g O 12.01 1.01 16.00 4.996 mole C4.44 mole H2.221 mole N 2.221 2.221 2.221 2.24921 x4x4x4 984 C9H8O4C9H8O4 Calculating Empirical Formula (special)

1.Find the empirical formula 2.Calculate the molar mass of your empirical formula 3.Identify the molar mass of your molecular (GIVEN in the problem everytime!) 4.Divide the molecular mass / empirical mass 5.Round to the nearest whole # 6.Multiply the whole # by the subscripts in the Empirical formula Calculating Molecular Formula

If a compound has an empirical formula of NO 3 and a molecular mass of 186g – what is the molecular formula? Empirical formula: NO 3 molar mass: 62.01g Molecular mass (given) 186g empirical mass 62.01 3 x NO 3 = N 3 O 9 Practice

For counting matter (quantity) in chemistry we use the mole Moles are used to correctly measure chemicals for reactions in a lab. Problem – no way to “physically” measure a mole. (no lab equipment measures moles) Solution – molar mass (balance) What is a mole?

Remember – Molar mass is the mass (grams) of 1 mole 1 mole Fe = _________grams Fe 2.5 mole Fe = ________ grams Fe 113.5 grams Fe = _______ moles Fe Mass to mole = divide by molar mass Mole to mass = moletiply by mole mass Using Molar Mass

How many grams are in 15.7 mole MgCl 2 How many moles are in 0.75 grams of silver? What is the mass of 30.7 mole water? Using molar mass

In the lab, Mrs. Mathieson needs 2.57 moles of NaCl to do an experiment. How many grams would be needed to equal 2.57 moles of NaCl? After doing the experiment, Mrs. Mathieson has 1.02 moles of NaCl remaining – how many grams does that equal? Using molar mass

For counting matter (quantity) in chemistry we use the mole 1 mole = 6.02 x 10 23 representative particles (particles are very tiny) This value is called Avogadro’s Number What is a mole?

ie. the smallest particle that retains chemical and physical properties 3 types depending on the compound : Atoms: Single element Molecules: covalent compound Formula Units: Ionic Compounds or ions What are REPRESENTATIVE PARTICLES ?

Remember there are 6.02 x 10 23 particles (atoms, molecule, f.u) in 1 mole How many atoms are in 1 mole? How many atoms are in 2.10 moles of Copper? How many moles are in 4.21 x 10 26 atoms of aluminum? Using Avagadro’s Number

For counting matter (quantity) in chemistry we use the mole 1 mole = 22.4 L of any gas This value is called Molar Volume What is a mole?

How many liters are in 1 mole of any gas? How many liters are in 12.95 L of oxygen gas? How many moles are in 0.758 moles of nitrogen gas? Using molar volume

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