Holy Moley: Moles as Measurements of matter Chapter 10 Holy Moley: Moles as Measurements of matter

A quick review of moles a mole is a specific number of particles (like a dozen is 12 particles) – it is Avogadro’s number of particles – 6.022 x 1023 particles the molar mass of a substance can be found using its formula, and is the mass, in grams, of a mole of the substance

determining chemical formulas Our lab information about a substance is usually a ratio of masses (in grams) for example, we might know that a sample of copper chloride is composed of 4.7 grams of copper and 5.3 grams of chlorine (4.7 g:5.3 g) Chemical formula information is a ratio of moles for example, the ratio of moles in a sample of water is 2 H’s:1 O Therefore, to determine the chemical formula when we have a ratio of masses, we must convert that ratio to moles

To determine the chemical formula Start with the relative mass of each element in a compound Divide each mass by the molar mass of that element (to determine relative number of moles) Divide all answers by the smallest answer to get a ratio of whole numbers These numbers are the subscripts in the chemical formula

Example #1 What is the empirical formula for a compound which contains 0.0134 g of iron, 0.00769 g of sulfur and 0.0115 g of oxygen?

Example #2 Find the formula for a compound which contains 32.8% chromium and 67.2% chlorine.

Practice A 0.810-gram sample is decomposed in the lab and found to contain 0.429 grams of aluminum. The remainder of the compound is oxygen. Determine the formula.

Practice A 5.0-gram sample of a white powder is decomposed and found to contain 1.11 grams of aluminum and 1.27 grams of phosphorus. The remainder of the compound is oxygen. What is the formula for this compound?

To determine the chemical formula Either relative masses OR percent compositions can be used If the mole ratios end up as decimals of 0.5 or 0.33, multiply all numbers by 2 or 3 to achieve all whole-numbers The chemical formula determined is the most simplified version of the formula

Percent Composition and Empirical vs. Molecular formulas

formula mass (Molecular Mass) The formula mass (or molecular mass) is the sum of the mass of each atom in the compound, in amu The molar mass of a compound is numerically equal to the formula mass, but with units of grams/mole The formula mass of any compound is the sum of the average atomic masses of all the atoms represented in the formula Determine the number of atoms of each element in the compound Multiply the number of atoms by the atomic mass of that element Add all answers to obtain the formula mass for the whole compound

examples H2SO4 Fe2(SO4)3 MgSO4•3H2O

Empirical vs. molecular formula The empirical formula of a compound is the chemical formula in its most simplified form. It tells you the simplified ratio of the elements in a compound. CH2O The molecular formula of a compound may or may not be the same as the empirical formula. It is the formula that tells the actual number of atoms of each element in the compound. C3H6O3 or C6H12O6

examples Determine the empirical formula for each of the following molecular formulas: B4H10 C6H12O6 N2O4 FeSO3 Write four possible molecular formulas that would all have the empirical formula C2H5.

Relationship Between Empirical and Molecular Formula Masses If an empirical formula and the molecular mass are known, the molecular formula can be found. Consider the empirical formula CH2O If the molecular mass is 90 amu, what must the molecular formula be? To determine the molecular formula- compare the formula mass of the empirical formula with the molecular mass given to see how many times greater the molecular mass is. Formula mass of CH2O = 30 amu, 30 x _____ = 90  Molecular formula is 3 times bigger! Molecular Formula is C3H6O3

Examples Acetylene is a colorless gas used as a fuel in welding torches, among other things. It is 92.96% carbon and 7.74% hydrogen. Its molecular mass is 26.02 amu. What are the empirical and molecular formulas of acetylene?

Calculating percent composition Percent composition of a compound is the percent by mass of each element in the compound Percent composition can be used to determine the empirical formula of an unknown compound Percent composition can also be used to determine the amounts of elements in a given sample

Calculating percent composition Calculate the formula mass of the compound using the previously given steps Determine the mass of the element in the compound by multiplying the atomic mass by the number of atoms of that element Use the following formula:

example What is the percent by mass of nitrogen in magnesium nitrate, Mg(NO3)2?

practice For each compound below, calculate the formula mass. Then calculate the percent composition of oxygen in the compound. H2O H2O2 KClO3 Ti3(PO4)4