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4.4 Formulas of Compounds

4.4 Formulas of Compounds. 4.4.1 Percentage Composition. Percentage Composition. The values for molar masses of elements in compounds can be used to calculate the percentage composition of compound once its formula is know.

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4.4 Formulas of Compounds

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  1. 4.4 Formulas of Compounds 4.4.1 Percentage Composition

  2. Percentage Composition • The values for molar masses of elements in compounds can be used to calculate the percentage composition of compound once its formula is know. • This type of calculation to obtain the percentage by mass of an element is important.

  3. % by mass of the element =

  4. Example Stannous fluoride, SnF2 is an active ingredient in some toothpastes. Find the percentage composition by mass of each element in the compound. One formula of SF2 contains one Sn atom and 2 F atoms: M(SnF2) = 118.7 + (2 × 19) = 156.7 Since Sn contributes 118.7 units towards this mass, % mass Sn = =75.7% we have 2 Flourine so 2 × 19 units, so we get % mass F = × =24.3%

  5. 4.4.2 Empirical Formulas • This is the formula that gives the simplest whole number ratio, number of moles, of each element in the compound. • Which of the following are empirical formulas? C2H8 or CH4?

  6. Steps to calculate Empirical Formulas • Determine the mass in grams of each element present, if necessary. • Calculate the number of moles of each element. • Divide each by the smallest number of moles to obtain the simplest whole number ratio. • If whole numbers are not obtained in step 3), multiply through by the smallest number that will give all whole numbers

  7. Determining an empirical formula given elemental composition • A compound of Sulfur contains 2.4% hydrogen, 39.0% sulfur and 58.6% oxygen. Find the empirical formula. • Symbols H S O • Masses 2.4g 39.0g 58.6g • Moles =2.4 =1.2 =3.7 • Ratio/divide by the smallest =2.0 =1.0 =3.1 • So empirical formula is H2SO3

  8. Questions • PT12 pg 69 Q14; Worksheet Q7abefg • PT13 pg 69, Q15; Worksheet Q11-f, 2-6

  9. Empirical formula of hydrated salts • Some ionic compounds crystallise from an aqueous solution to form a hydrated ionic compound. In these compounds, water molecules are included in the crystal lattice structure. This water is called water of crystallisation. • Hydrated copper (II) sulfate, for examples, appears as a blue crystalline solid and has the formula CuSO4.5H2O. • Heating a solution of this will remove the water molecules to leave a white powder known as anhydrous copper(II) sulfate.

  10. Determining the empirical formula of a hydrated ionic compound. • Washing soda crystals may be used to bleach linen. When crystallised from water, washing soda (sodium bicarbonate, Na2CO3) forms crystals of a hydrated ionic compound. When 5.0g of washing soda crystals were dried in a desiccator, 1.85g of sodium carbonate remained. Calculate the empirical formula of the hydrated compound. • Calculate the mass of water present in the hydrated compound. Since 1.85g of Na2CO3 was obtained on dehydration of the crystals, then the amount of water is 3.15g, or the difference in mass between the hydrated and dehydrated compound (5-1.85=3.15g) • The mole ratio of Na2CO3 to water is: = 0.0175: 0.175 1:10 • So the empirical formula of the washing soda crystals is Na2CO3.10H2O

  11. 4.4.3 Molecular Formulas • The empirical formula gives the simplest whole number ratio of each element in a compound, a molecular formula gives the actual number of atoms in one molecule of the compound.

  12. The empirical formula of hydrogen peroxide is HO and its relative molecular mass is 34.0. Find the molecular formula. Empirical formula is HO = 17gmol-1 But Molar mass is = 34.0 gmol-1 Number of OH units in 1 molecule = = 2 So Molecular formula = 2 × empirical formula; so the molecular formula is: H2O2

  13. Napthalene has an empirical formula of C5H4 and 0.2 mol of the compound has a mass of 25.6g. Find the molecular formula. Empirical formula is C5H4 = 64 Molar mass of compound = ? m(compound) = 25.6g and we have 0.2 mole so we can work out the Molar mass of the compound. n = so M = which is: gmol-1 Number of C5H4 units present = = 2 So Molecular formula is: C10H8

  14. The common insect repellent sold commercially as ‘mothballs’ is the organic compound naphthalene. It is a hydrocarbon containing 93.7% carbon and 6.3% hydrogen and has a molar mass of 128g mol-1. Find the empirical and molecular formulae of naphthalene.

  15. Questions • PT14 Worksheet Q1 h,i • PT 15 Pg 69 Q 16, 17, 18; pg 71 Q44, 49 • Review Question (Recommended) • Worksheet “Relative Atomic Mass & the Mole – Revision” • Page 70-72 Selected problems as per handout.

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