Compounds that Contain the Same Elements in Different Proportions
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Chemical Proportions and Percentage Composition 1 Compounds Compounds That Contain the Same Elements in the Same Proportions Water on Earth has the same chemical composition as water on the Moon. More than 200 years ago, scientists discovered that compounds contain elements in fixed proportions. Water ALWAYS has 2 hydrogen atoms and 1 oxygen atom. This discovery was accepted into law almost 100 years after it was made The Law of Definite Proportions The elements in a chemical compound are always present in the same proportions by mass. Mass Percent According to the law, all water molecules have the same chemical composition. Therefore, 1 mol of water has a mass of 18.02g and contains 16.00g of oxygen and 2.02g of hydrogen. However, the law deals with proportions—not mass. As a result, the proportion of each element is often expressed as a percentage of the total mass. This is known as mass percent. Calculating Mass Percent Using the Chemical Formula Determine the mass percent of hydrogen in water: πππ π πππππππ‘ ππ π» = πππ π ππ π» × 100% πππ π ππ π»2 π πππ π πππππππ‘ ππ π» = 2.02π × 100% 18.02π πππ π πππππππ‘ ππ π» = 11.3% Calculating Mass Percent Using Experimental Data A 650 mg sample was analyzed and found to contain 51.0 mg of hydrogen. What is the mass percent of the sample? πππ π πππππππ‘ ππ π» = πππ π ππ π» × 100% πππ π ππ π πππππ πππ π πππππππ‘ ππ π» = 51.0π × 100% 650π πππ π πππππππ‘ ππ π» = 7.84% Chemical Proportions and Percentage Composition 2 Compounds Compounds that Contain the Same Elements in Different Proportions All water has the same chemical formula—H2O. But there are hundreds of compounds that contain the same elements, but in different proportions. For example, H2O and H2O2. Water is a stable compounds, whereas hydrogen peroxide is quite reactive. NO2 and N2O are two very different compounds, despite containing the same elements. Nitrogen dioxide is a product of car exhaust is highly toxic. Dinitrogen monoxide (aka “laughing gas”) is routinely used to calm patients during dental procedures. Different Proportions = Different Properties Percentage Composition When you combine the mass percent for all of the elements in a compound, you have the percentage composition of the compound. Percentage Composition of NO2(g) Percentage Composition of N2O(g) Mass percent of N = 30.45% mass percent of N = 63.65% Mass percent of O = 69.55% mass percent of O = 36.25% Determining Percentage Composition from Mass Data A volatile chemical was removed from a crime scene. Analysis of a 35.20 mg sample of the chemical showed that it contained 3.56 mg of carbon and 0.28 mg of hydrogen. Later, investigators learned that the chemical was chloroform, CHCl3(l). What is the percentage composition of chloroform? mass of total sample: 35.2 mg mass of carbon sample: 3.56 mg mass of hydgrogen sample: 0.28 mg 1. Calculate the mass of the chlorine sample πππ π ππ πβππππππ = πππ π ππ π πππππ − πππ π ππ ππππππ − πππ π ππ βπ¦ππππππ πππ π ππ πβππππππ = 35.20ππ − 3.56ππ − 0.28 ππ πππ π ππ πβππππππ = 31.36ππ 2. Determine the percentage composition of each element πππ π πππππππ‘ ππ ππππππ = πππ π ππ πΆ × 100% πππ π ππ π πππππ Chemical Proportions and Percentage Composition 3 Compounds πππ π πππππππ‘ ππ ππππππ = 3.56ππ × 100% 35.20ππ πππ π πππππππ‘ ππ ππππππ = 10.11% Repeat for hydrogen and chlorine: - Mass percent of hydrogen: 0.80% Mass percent of chlorine: 89.09% Determining Percentage Composition from a Chemical Formula Given the chemical formula, any mass for the sample sized could be picked. For convenience, 1 mol of the substance is used as the sample size. Determine the mass percent of Cu in copper (I) sulfide, Cu2S. πππ π πππππππ‘ ππ πΆπ’ = πππ π ππ πΆπ’ ππ 1 πππ ππ πΆπ’2 π × 100% πππ π ππ 1 ππππ ππ πΆπ’2 π πππ π πππππππ‘ ππ πΆπ’ = 2 × 63.55π × 100% 159.17π πππ π πππππππ‘ ππ πΆπ’ = 79.9% Empirical and Molecular Formulas The molecular formula of a compound shows the number of atoms of each element that make up a molecule. For example, H2O and C6H12O6 (glucose). The empirical formula of a compound shows the smallest whole-number ratio of the elements in a compound. For example, CH2O (glucose). Chemical Proportions and Percentage Composition 4 Compounds The empirical formula gives a very basic glimpse into the composition of a compound—and is therefore a very good starting point. Determining the Empirical Formula 1. Convert percentage composition data into mass data assuming that the total mass of the sample is 100g. 2. Determine the number of moles of each element in the sample by dividing the mass by the molar mass of each element. 3. Convert the number of moles of each element into subscripts. 4. Convert the number of moles of each element into whole numbers by dividing each subscript by the smallest amount. 5. If the subscripts are not yet whole numbers, determine the least common multiple that will make the decimal values into whole numbers. Multiply all subscripts by this common multiple. Example: Determine the empirical formula for a compound that is found by analysis to contain 27.37% sodium, 1.200% hydrogen, 14.30% carbon and 57.14% oxygen. Chemical Proportions and Percentage Composition 5 Compounds Determining the Molecular Formula Because the molecular formula is so close to the empirical formula, you can determine the molecular formula of a compound by using the empirical formula and the molar mass of the compound. 1. Determine the molar mass of the empirical formula 2. Use the given molar mass of the compound (mass spectrometer) to determine the whole number multiplier. Ex. The empirical formula for glucose is CH2O(s). The molar mass of glucose is 180.18 g/mol. Determine the molecular formula for glucose. 1. Determine the molar mass of CH2O(s) ππΆπ»2 π = ππΆ + 2ππ» + ππ ππΆπ»2 π = 12.01π 1.01π 16.00π + 2( )+ πππ πππ πππ ππΆπ»2 π = 30.03π πππ 2. Determine the whole number multiplier: ππππ‘π’ππ ππππππ’ππ = π₯ × ππΆπ»2 π π₯= 180.18π/πππ 30.03 π/πππ π₯=6 3. Multiply all the subscripts of the empirical formula by 6 Molecular formula of glucose is C6H12O6
