Chemical Quantities Chapter 10

The Mole: A Measurement of Matter We can measure mass (g), volume (L), count atoms or molecules in MOLES Pair: 1 pair of shoelaces (2) ; Dozen: 1 dozen oranges (12); Ream: 1 ream of paper (500) 1 mole = 6.02 x of the representative particles. Mole is an amount, defined as the number of carbon atoms in exactly 12 grams of carbon-12.

The Mole: A Measurement of Matter 6.02 x is called: Avogadro’s number. What are representative particles ? The smallest pieces of a substance: 1)For a molecular compound: it is the molecule. 2)For an ionic compound: it is the formula unit (made of ions). 3)For an element: it is the atom.

Questions 1 mole = 6.02 x of the representative particles. How many molecules of CO 2 are in 4.56 moles of CO 2 ? Known: 4.56 mol CO 2 Unknown: molecules of CO 2 Relationship: 1 mol CO 2 = 6.02 x molecules of CO 2 Conversion Factors 4.56 mol CO 2 X = 25.4 X = 2.54 X m olecules 1 mol 6.02 x molecules 1 mol 6.02 x molecules 1 mol

The mass of one mole of any substance is called the molar mass. (same value as the atomic mass) ie: Molar mass of He = Molar mass of Hg = Molar mass of Li = 4 g/mol g/mol 6.94 g/mol

Examples  Calculate the molar mass of the following compounds Na 2 S N2O4 N2O4 C Ca(NO 3 ) 2 C 6 H 12 O 6 (NH 4 ) 3 PO 4 = 78 g/mol = 92 g/mol = 12 g/mol = 164 g/mol = 180 g/mol = 149 g/mol

The Mole – Mass Relationship Conversion factors : To change between grams of a compound and moles of a compound. Molar mass = 1 mole molar mass 1 mol molar mass and MASS IN GRAMS MOLES NUMBER OF PARTICLES Divide with molar mass Multiply with molar mass multiply with Avogadro’s number divide with Avogadro’s number

If you are given moles, you……………MOLE- TIPLY!

How many moles is 5.69 g of NaOH ? need to change grams to moles for NaOH 1mole Na = 22.99g ; 1 mol O = g ; 1 mole of H = 1.01 g 1 mole NaOH = g

Many of the chemicals we deal with are in the physical state as: gases. They are difficult to weigh (or mass). But, we may still need to know how many moles of gas we have. Two things effect the volume of a gas: a) Temperature and b) Pressure We need to compare all gases at the same temperature and pressure. The Mole – Volume Relationship

Standard Temperature and Pressure (STP) - means a temperature of 0 o C and pressure of 1 atm. At STP, 1 mole of any gas occupies a volume of 22.4 L Called the molar volume Relationship : 22.4 L = 1 mol at STP 22.4 L 1 mol 22.4 L and

Lets try these examples :  What is the volume of 4.59 mole of CO 2 gas at STP?  How many moles is 5.67 L of O 2 at STP?  What is the volume of 8.8 g of CH 4 gas at STP? = 103 L = mol = 12.3 L

Density of Gas D = m / V (density = mass/volume) -for a gas the units will be: g / L We can determine the density of any gas at STP if we know its formula. To find the density we need: 1) mass and 2) volume. D = m / V

Density of Gas Lets assume we have 1 mole of gas, then the mass is the molar mass (from periodic table) And, at STP the volume is 22.4 L. 1. Find the density of CO 2 at STP 2. Find the density of CH 4 at STP  3. What is the molar mass of a gas with a density of  4. How about a density of 2.86 m = D x V

Percent Composition of a Compound The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100. % by mass of element = mass of element mass of compound × 100

Percent Composition of a Compound Find the % composition of Cu 2 S %Cu = g S g Cu 2 S  100 = % Cu %S = g Cu g Cu2S  100 = 20.15% S

Determine the percent by mass of C in C 3 H 8. % C = mass of C in 1 mol C 3 H 8 molar mass of C 3 H 8 × 100% = 81.8% C = × 100% 36.0 g 44.0 g Calculate Solve for the unknowns.

Determine the percent by mass of H in C 3 H 8. % H = mass of H in 1 mol C 3 H 8 molar mass of C 3 H 8 × 100% = 18% H = × 100% 8.0 g 44.0 g Calculate Solve for the unknowns.

The percents of the elements add up to 100% when the answers are expressed to two significant figures (82% + 18% = 100%). Evaluate Does the result make sense?

You can also calculate the percent composition of a compound using its chemical formula. CuSO 4. 5 H 2 O ( hydrated compounds) Percent Composition from the Chemical Formula Mass of molecule in sample of compound Total mass of compound x 100

H 2 0: 5 x 18 = 90 Cu: 1 x 63.5 S: 1 x 32.0 O: 4 x 16.0 Mass of molecule in sample of compound = 90 Total mass of compound = = = 63.5 = 32.0 = /249.5 = x 100 = 36.07% CuSO 4. 5 H 2 O