Mole problems with compounds

Mole calculations with elements How many grams are equal to 7.5 x 1022 atoms of boron? How many moles are equal to 2.5 grams of helium?

What about mole calculations with compounds?

If we’re talking about water, what do you think 1 mole equals?

If we’re talking about water, what do you think 1 mole equals? 6.02 x 1023 molecules

If we’re talking about sodium chloride, what do you think 1 mole equals?

If we’re talking about sodium chloride, what do you think 1 mole equals? 1 mole = 6.02 x 1023 formula units (smallest piece of a crystal lattice; NaCl)

Molar Mass The molar mass concept works the same way with compounds as it did with pure elements. You simply add the molar mass of each atom within the formula. We call this “molar mass” for molecular compounds and “formula mass” for ionic compounds. Generally, though, we use the term molar mass for atomic mass in grams of any compound, ionic or covalent, or any element. Where do you find the molar mass of an element?

Molar Mass of Compounds Water -H2O Salt-NaCl 2(1.008) + 15.999 = 18.015 g/mol 22.990 + 35.453 = 58.443 g/mol

Molar Mass Examples Sodium bicarbonate NaHCO3 22.990 + 1.008 + 12.011 + 3(15.999) = 84.006 g/mol Sucrose C12H22O11 12(12.011) + 22(1.008) + 11(15.999) = 342.297 g/mol

Practice Calculate the molar mass of: CaCl2 Carbon monoxide

If parentheses appear in a formula, the number outside the parenthesis multiplies by every atom in the parenthesis, just like a coefficient in math. Ca(NO3)2 = 1 calcium, 2 nitrogen, 6 oxygen Molar Mass = 40.08 g + 2(14.007) + 6(16.00) = _______.

Write the formula of the following compounds Write the formula of the following compounds. How many of each atom are in these compounds? Sodium Phosphate Magnesium Phosphate Aluminum Phosphate

Calculate the molar mass of each compound. 6. Calculate the molar mass of Hg3(PO4)2 7. Calculate the molar mass of zinc nitrate. 8. Calculate the molar mass of magnesium hydroxide.

Remember…. 1 mole (of an element) = 6.02 x 1023 atoms (of that element) = molar mass in grams (of that element) This now applies to compounds as well. Instead of atoms, we refer to molecules (molecular compounds) or formula units (ionic compounds). 1 mole (of any substance) = 6.02 x 1023 atoms, molecules, formula units= molar mass in grams (of that substance)

Practice 1. How many moles are equal to 5.06 x 1023 molecules of Br2?

How many moles are equal to 1.50 x 1022 molecules of ammonia, NH3?

3. How many moles are equal to 3 3. How many moles are equal to 3.905 x 1023 formula units of calcium hydroxide?

4. What is the mass of 2.44 moles of sodium hydroxide?

5. How many moles are equal to 5.96 grams of magnesium acetate?

6. How many nitric acid molecules are in 4.20 g of HNO3?

Percent Composition % composition = the % by mass of a given element in a compound

Percentage Composition % composition = molar mass of element x 100 molar mass of compound

Percentage Composition Find the molar mass of each element in the compound. Multiply by how many molecules of each element. Divide by the total molar mass of the compound. Multiply by 100. Hydrogen: 2(1.008)= 2.016 Oxygen: 1(15.9994)=15.994+ Ex: Calculate the % hydrogen in water. 18.015 Chemical formula: H2O 1.008(2) 1.008(2) + 15.999 x 100 x 100 = 11.19 % hydrogen

Percentage Composition Find the % composition of Cu2S. 127.092 g Cu 159.160 g Cu2S %Cu =  100 = 79.8517% Cu 32.068 g S 159.160 g Cu2S %S =  100 = 20.148% S

Percentage Composition How many grams of copper are in a 38.0-gram sample of Cu2S? Cu2S is 79.8517% Cu (38.0 g Cu2S)(0.798517) = 30.3 g Cu

Practice What is the % oxygen in chlorine trioxide? Chemical formula: ClO3

Practice 4. Calculate the % composition of all elements in sodium phosphate. What do you notice about all of the %s?

Hydrates Hydrates are ionic compounds that can trap water in their crystalline structure when they form. The water is a part of the structure, and it is a definite ratio of the compound. Examples: CuSO4·5H2O copper (II) sulfate pentahydrate Na2CO3·10H2O sodium carbonate decahydrate

Write the formula for magnesium sulfate heptahydrate.

Anhydrates Anhydrate compounds have no water in their crystalline structure. Example: NaCl

To calculate the % water in a hydrate, use the same formula as before, but water is your part on top. % composition = # of water molecules(MM of water)/mm of hydrate compound x 100%

1. Calculate the % water in sodium carbonate decahydrate.

2. Calculate the % water in magnesium sulfate heptahydrate.

Empirical & Molecular Formulas

C2H6 CH3 Empirical Formula reduce subscripts Smallest whole number ratio of atoms in a compound All ionic compound formulas are empirical formulas C2H6 reduce subscripts CH3

Empirical Formula % to mass Mass to moles Divide by least Multiply until whole

Empirical Formula 1. Find the empirical formula for a sample of 25.9% N and 74.1% O. 25.9 g N 1 mol 14.007 g N = 1.85 mol N = 1 N 1.85 mol 74.1 g O 1 mol 15.999 g O = 4.63 mol O = 2.5 O

N2O5 N1O2.5 Empirical Formula Need to make the subscripts whole numbers  multiply by 2 N2O5

A compound was analyzed and found to contain 13. 5 g Ca, 10 A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H.  What is the empirical formula of the compound?

Lab Results (elements listed as % composition) 46.9% Na (Sodium) 24.5% C (Carbon) 28.6% N (Nitrogen) 3. What is the empirical formula? NaCN (sodium cyanide)

HO H2O2 Molecular Formula “True Formula” - the actual number of atoms in a compound HO empirical formula ? H2O2 molecular formula

Molecular Formula— you must know/be given the molar mass of the molecule 1. Find the empirical formula. 2. Find the molar mass of the empirical formula. 3. Divide the molecular mass (given) by the empirical mass. 4. Multiply each subscript in the empirical formula by the answer from step 3.

Molecular Formula A compound has the empirical formula “CH.” The molar mass of the compound is 78.110 g. What is the molecular formula of the compound?

Practice problem 2. The % composition of caffeine is 49.5 % carbon, 5.20 % hydrogen, 16.5 % oxygen, and 28.9 % nitrogen. What is the molecular formula if the molar mass of caffeine is 194.1 g?