MATH OF CHEMISTRY
Compounds are electrically neutral. A compound is a substance composed of two or more different elements that are chemically combined in a fixed proportion. A chemical compound can be broken down by chemical means. Compounds are electrically neutral. J Deutsch 2003
The subscript 1 is never written in a formula A chemical formula is both qualitative and quantitative. It tells which elements are in the compound with symbols and how many of each with subscripts. The formula for sulfuric acid is H2SO4 Element Number of atoms H 2 S 1 O 4 The subscript 1 is never written in a formula J Deutsch 2003
The formula mass must be a whole number multiple of the empirical mass Molecular Empirical Multiple Formula Mass Formula Mass H2O2 34 HO 17 2 C3H6 42 CH2 14 3 C6H12O6 180 CH2O 30 6 Hg2O2 434 HgO 217 2 J Deutsch 2003
The formula mass of a substance is the sum of the atomic masses of its atoms. The molar mass (gram formula mass) of a substance equals one mole of that substance. (3.3e) Formula mass of H2SO4 H 2 x 1 = 2 S 1 x 32 = 32 O 4 x 16 = 64 98 amu J Deutsch 2003
The formula mass represents the mass of one molecule of a substance while the gram formula mass represents the mass of a mole (6.02 x 1023 molecules) of that substance. The calculation for formula mass and gram formula mass (GFM) are the same, the difference is in the units. Formula Mass of H2SO4 = 98 amu Gram Formula Mass of H2SO4 = 98 g If they tell you to use the mass rounded to the nearest tenth then use tenths for each mass as well as your final answer. J Deutsch 2003
In all chemical reactions there is a conservation of mass, energy, and charge. Matter cannot be created nor destroyed, only changed from one form to another Energy cannot be created nor destroyed, only changed from one form to another J Deutsch 2003
A balanced chemical equation represents conservation of atoms. A mole of molecules is made up of 6.02 x 1023 molecules A chemical equation is balanced to ensure the conservation of matter/energy, and charge. The total mass before the reaction must equal the total mass after the reaction has taken place. J Deutsch 2003
Chemical equations must be balanced so that mass can be conserved. Word equation: hydrogen + oxygen water Chemical equation: 2H2 + O2 2H2O J Deutsch 2003
Regents Question: 08/02 #9 If an equation is balanced properly, both sides of the equation must have the same number of (1) atoms (2) coefficients (3) molecules (4) moles of molecules þ 2H2 + O2 2H2O J Deutsch 2003
How many on the left? How many on the right? What do I do? Balance the reaction Fe + O2 Fe2O3 There are 2 oxygen atoms on the left and 3 on the right. To get them equal I need to multiply the left by 3 and the right by 2. These multiples are called coefficients and are placed in front of the formula. The yield sign and the plus separate the formulas. Fe + 3O2 2Fe2O3 Now the number of iron atoms has to be balanced. There is one iron on the left and four on the right. Use a coefficient of 4 in front of the Fe on the left. 4Fe + 3O2 2Fe2O3 Now it is correctly balanced J Deutsch 2003
Regents Question: 06/03 #59 Given the reaction between two different elements in the gaseous state Box A below represents a mixture of the two reactants before the reaction occurs. The product of this reaction is a gas. Draw the system after the reaction has gone to completion, based on the Law of Conservation of Matter. One reactant is in excess. Box A – System before reaction Box B – System after reaction J Deutsch 2003
Use the coefficients to predict the amount of reactant consumed or product formed (Mole-Mole problems) The ratio of the coefficients is a ratio of the moles taking part in a reaction. Reactants (on the left of the arrow) are consumed Products (on the right of the arrow) are formed Given the number of moles of any substance in a reaction, you can use the coefficients to find the number of moles of any other substance. J Deutsch 2003
Place the moles given over the coefficient for that substance and set up a proportion with the coefficients. How many moles of oxygen are produced when 4 moles of KClO3 react according to the equation: 2 KClO3 2 KCl + 3 O2 4 X 2 KClO3 2 KCl + 3 O2 4 = X 2 3 cross multiply to get 2X = 12 X=6 moles unknown given J Deutsch 2003
Regents Question: 01/03 #42 þ Given the equation: 2C2H2(g) + 5O2(g) 4 CO2(g) + 2H2O(g) How many moles of oxygen are required to react completely with 1.0 mole of C2H2? 2.5 2.0 5.0 10 þ J Deutsch 2003
Regents Question: 06/02 #37 þ Given the reaction: 6CO2 +6H2O C6H12O6 + 6O2 what is the total number of moles of water needed to make 2.5 moles of C6H12O6 ? 2.5 6.0 12 15 þ J Deutsch 2003
Regents Question: 06/03 #20 þ Given the reaction: PbCl2 (aq) + Na2CrO4 (aq) PbCrO4 (s) + 2NaCl(aq) What is the total number of moles of NaCl formed when 2 moles of Na2CrO4 react completely? (1) 1 mole (2) 2 moles (3) 3 moles (4) 4 moles þ J Deutsch 2003
Regents Question: 06/02 #34 A compound has a gram formula mass of 56 grams per mole. What is the molecular formula for this compound? CH2 C2H4 C3H6 C4H8 þ J Deutsch 2003
Regents Question: 06/02 #41 þ The gram formula mass of NH4Cl is 22.4 g/mole 28.0 g/mole 53.5 g/mole 95.5 g/mole N 1 x 14.0 = H 4 x 1.0 = Cl 1 x 35.5 = _______ g/mole þ J Deutsch 2003
Grams/GFM = Moles To convert grams into moles, divide the grams by the gram formula mass To convert moles into grams, multiply by the moles by the gram formula mass The unit for gram formula mass is grams/mole How many moles in 49 grams of H2SO4? 49 g = X moles X= 0.50 moles 98 g/mol How many grams are contained in 2.00 moles of H2SO4? X = 2.00 moles X = 196 grams J Deutsch 2003
The percent composition by mass of each element in a compound can be calculated mathematically. (3.3f) Calculate the formula mass and then divide the component of the element you are looking for by the total mass of the formula then multiply by 100%. Formula mass of H2SO4 % of Oxygen in H2SO4 H 2 x 1 = 2 64/98 x 100% = 65% S 1 x 32 = 32 O 4 x 16 = 64 98 J Deutsch 2003
Regents Question: 01/03 #8 What is the percent by mass of oxygen in H2SO4? [ formula mass = 98] 16% 33% 65% 98% 64/98 x 100 þ J Deutsch 2003
Regents Question: 06/02 #7 The percent by mass of hydrogen in NH3 is equal to 17 x 100 1 (2) 17 x 100 3 1 x 100 17 (4) 3 x 100 þ J Deutsch 2003
Regents Question: 06/03 #10 The percent by mass of calcium in the compound calcium sulfate (CaSO4 ) is approximately 15% (2) 29% (3) 34% (4) 47% þ J Deutsch 2003
A hydrate is a compound which has water trapped in its crystal structure. We can determine the percentage of water in a hydrate. CuSO45H2O – for every copper(II) sulfate there are 5 trapped water molecules Find the formula mass including the water Cu 1 x 64 = 64 S 1 x 32 = 32 O 4 x 16 = 64 H 10 x 1 = 10 Water contributes 90 to O 5 x 16 = 80 the 250 250 90/250 x 100% = 36% J Deutsch 2003
Regents Question: 06/02 #36 What is the total number of oxygen atoms in the formula MgSO4•7H2O? [The • represents seven units of H2O attached to one unit of MgSO4] 11 7 5 4 þ J Deutsch 2003