CHEMICAL QUANTITIES Chapter 6 SCH 3U

Law of Definite Proportions What is the chemical formula of Martian water? What is the chemical composition of lunar water?

Law of Definite Proportions ■The elements in a chemical compound are always present in the same proportions by mass.

Law of Definite Proportions ■Consider the molecules H 2 O H 2 O 2 Both molecules contain hydrogen and oxygen but in different proportions. This creates very different properties. Can you think of some others?

Mass Percent ■We express the chemical composition of a substance as a mass percent of each element in the compound. ■The mass percent of an element is the mass of one element within a compound expressed as a percentage of the total mass of the compound.

Think about this for a minute… A company manufactures gift boxes that contain two pillows and a gold brick. The gold brick has a mass of 20kg and each pillow has a mass of 1.0kg. You are a quality control specialist at the gift box factory. Your job is to inspect each shipment of gift boxes. You need to know: 1)The percent of pillows in the gift box. 2)The percent of bricks in the gift box. 3)The percent of pillows in a truckload of gift boxes. 4)The percent of bricks in a truckload of gift boxes.

Percentage Composition of a Substance ■The percentage composition of a substance refers to the combined mass percent of all the elements present in a compound. ■This can be calculated from experimental data, measuring mass or by molar mass data if the formula is known.

Examples ■A sample of a compound was analyzed and found to contain 7.20 g of carbon, 2.20 g of hydrogen and 17.6 g of oxygen. Calculate the percentage composition of the compound.

Examples ■Determine the percentage composition of sodium sulphate, Na 2 SO 4

What makes popcorn pop? ■Have you ever wondered what the optimal percentage of water in a popcorn kernel is? ■Design an experiment to find out and calculate the mass percent of water in a popcorn kernel!

Time to Work… Page 262 #4, 5, 6 Page 264 #12, 16 Page 266 #23, 26, 27, 29

Empirical and Molecular Formula Percentage composition of a compound only tells a chemist the elements that are present and the relative masses of these elements. It does not provide information as to the number of atoms of each element that are present.

Compare these molecules… NameMolecular Formula Empirical Formula Whole Number Multiple Use or Function FormaldehydeCH2OCH2OBiological preservative Acetic acidC2H4O2C2H4O2 Vinegar (5% solution) Lactic acidC3H6O3C3H6O3 Forms in muscles during exercise GlucoseC 6 H 12 O 6 Major nutrient for cells What do all of these compounds have in common?

■Empirical formula - expresses the relative number of atoms of each element present in the smallest whole number ratio; a formula unit of a compound. ■Molecular Formula – shows the actual number of atoms of each element in a molecule of the compound.

■Molecular formulas are whole number multiples of empirical formulas. ■Note that for some compounds the empirical and molecular formula are the same. ■Ionic compounds DO NOT have molecular formulas! Can you explain why?

Finding an empirical formula…

Calculate the empirical formula of a compound which shows 2.2% H, 26.7% C and 71.1% O.

The percentage composition of a fuel is 81.7% carbon and 18.3% hydrogen. Find the empirical formula of the fuel.

To find a molecular formula… 1.Determine the empirical formula of the compound. This may or may not be given in the question. 1.Calculate the molar mass of the empirical formula. 2.Divide the mass of the sample (given) by the molar mass of the empirical formula to find the whole number multiple. 3.Multiply the subscripts of the empirical formula by the whole number multiple to get the molecular formula.

CH 2 O is the empirical formula of a certain compound whose molecular mass is 180 u. What is the molecular formula of the compound?

The analysis of a compound shows that it is made up of 21.9% Na, 45.7% C, 1.9% H and 30.5% O. What is the molecular formula of the compound if its molecular mass is 210u?

Time to work… pg 270 #8, 9, 11, 12 pg 273 # 32, 34, 36, 39 pg 275 #43, 44, 46, 47