10 –3 %Composition, Empirical & Molecular Formulas

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Presentation transcript:

10 –3 %Composition, Empirical & Molecular Formulas

Percent Composition The percent by mass of each element in a compound. Example: Determine the % composition of each element in NaCl.

Practice % Composition Determine the % composition of each element in the following compounds: 1. Na2SO4 2. CaO 3. Al2(CO3)3

Hydrates Hydrates – compounds that contain water. Example : MgSO4  5H2O The ()really is equivalent to (+). Determine the formula mass of this compound.

% composition of hydrates Determine the % of water in MgSO4  5H2O. Determine the % of water in CaCO3  10H2O.

% Composition of Data Determine the percent composition of a 5.50g compound that contains sulfur and 3.30g of silver. 1st determine the amount of sulfur:

Using Percent composition. Determine the mass of an element in a given quantity of a compound. Multiply the % of the element in decimal form by the total mass of the compound. Example: Determine the mass of Ag in 120g of AgCl.

Practice % by mass 1. Determine the mass of Ca, in 400g of CaCO3. 2. Determine the mass of O, in 125g of H2SO4. 3. Determine the mass of K, in 50g of KClO34H2O.

Determining Chemical formulas Empirical Formula (Simplest Formula)– Smallest whole number ratio of atoms in a chemical formula.

Calculating an Empirical Formula: 1. Take the given % and change to grams. ex. 25.65 % = 25.65g 2. Convert each value to moles. (divide by molar mass) 3. Divide all answers by the smallest value. 4. If all answers are whole numbers, round to the nearest (+/- .1 ). Write the formula with those subscripts. 5. If not, then multiply each by a whole number to give a whole number value for all elements. ex. 1.5 x 2 = 3, or 1.4 x 5 = 7

Sample Problem Determine the simplest formula for a compound that contains 26.56% K, 35.41% Cr, 38.08% O.

Practice Problem Find the simplest formula for a compound that contains 32.38% Na, 22.65% S, 44.99% O.

Calculating the Molecular Formulas Molecular Formula – is a multiple of an empirical formula. Molecular mass is always in the given problem. Molecular Formula = (Empirical Formula)x

Sample Problem Determine the molecular formula of a compound having a simplest formula of CH and a formula mass of 78.110g CH, empirical mass = 13g

Sample Problem #2 A compound with a formula mass of 34g is found to consist of .44g H and 6.92g O. What is its molecular formula? 1st determine the empirical formula and its mass.

Assignment Page 344 -345 Group A Problems Group B Problems # 44 - 45 # 55

Review Problem #44. Find the empirical formula for a compound which contains 6.5g K, 5.9g Cl and 8.0g O.

Review Problem #45. The explosive, TNT, is composed of 37.0% C, 2.20% H, 18.5% N and 42.3% O. a) Determine the empirical formula for TNT. b) The molar mass for TNT is 227g/mol. What is the molecular formula for TNT?

Review Problem #55. Phenylalanine is an essential amino acid whose chemical composition is 65.5% C, 6.67% H, 8.48% N and 19.4% O. What is the empirical formula for phenylalanine?