The Mole Chemistry 11
Atomic Masses The atoms of different elements have different masses: Since the mass of an atom is very small, we use a special unit to describe it
Unified Atomic Mass Unit One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or a neutron) The unified atomic mass unit (U) or dalton (Da) is based off the weight of carbon. 1 U = 1/12 the mass of C- For Example: 12 The Magnesium atom has 12 protons, 12 neutrons, and 12 electrons. Since protons and neutrons have the same mass of approximately 1U, the atomic mass of Magnesium is 24.3U
Molecular Masses If we add up the masses of ALL the atoms that make up a molecule or a formula unit, we can calculate the atoms MOLECULAR MASS. For Example: Consider NaCl 1 x Na = 23.0 U 1 x Cl = 35.5 U 1 x NaCl = 58.5 U
Try some KCl H 2 SO 4 C 12 H 22 O 11 (NH 4 ) 2 SO 4
Avogadro s Number and the Mole When I buy eggs, how many come in a carton? How many cans of pop in a case? How many doughnuts in a box?
The MOLE is kind of like a dozen in Chemistry. The concept of the mole was proposed in the 1700 s by Amadeo Avogadro, an Italian scientist.
The measure of a mole is always the same: 1 mole = 6.02 x 10 23 atoms, molecules, ions, whatever! Because chemists deal with such large amounts of particles, it is easier to work with moles of particles. What makes the mole useful is that we can use it to convert between the mass of an element (in grams) and the number of atoms present.
The MOLAR MASS is the mass of one mole of a substance and is equal to the atomic mass, or molecular mass, expressed in grams. More accurately, we can say: The number of grams per mole of a substance written as g/mol.
For Example: 1 MOLECULE of O 2 = 32.0 U 1 MOLE or 6.02x10 23 atoms of O 2 = 32.0 g Could be written as 32.0 g/mol
How big is a mole? 1 Mole of marshmallows would be enough marshmallows to make a 19km thick layer of marshmallows covering the entire face of the Earth 1 mole of popcorn kernels could be spread uniformly over the USA if the thickness of the layer was about 14 km.
If I won a mole of dollars in the lottery that would be equal to: $602,000,000,000,000,000,000,000.00 (602 sextillion dollars)
Find the molar mass of the following: Ca 3 (PO 4 ) 2 (NH 4 ) 2 C 2 O 4 Ca(CH 3 COO) 2
Homework: Do: W.S. 10-1 W.S. 4-1 #1, 2
Using the Mole Using Avogadro s number and what we ve learnt from dimensional analysis, we can now do
Determine the mass of a substance given the number of moles You must determine the mass present, given the number of MOLES of a substance or vice versa. Use the following conversion factors:
For Example: What is the mass in grams of 3.25 mol of CO 2? How many moles of CH 3 OH are there in 0.250g of CH 3 OH?
Conversions between numbers of particles and moles of particles You must determine the number of moles that are present, given the number of particles present. Use the following conversion factors:
For Example: How many moles of Carbon are there in 3.01 x 10 24 C atoms? How many molecules does 3.65 moles of Nitrogen dioxide contain?
Conversions between number of moles and the volume of a gas Calculations involving gas volumes are simplified by Avogadro s Hypothesis:
The MOLAR VOLUME of a gas is the volume occupied by one mole of gas at STP = Temperature: 273 K (0 0 C) Pressure: 101.3 kpa
1 mol of ANY GAS at STP has the volume of 22.4 L In other words, the MOLAR VOLUME of any gas at STP is 22.4 L. Use the following conversion factor: Note : You may only use this conversion factor for a GAS at STP
For Example: What is the volume occupied by 0.350 mol SO 2(g) at STP? How many moles of gas are contained in a balloon with a volume of 10.0 L at STP?
Multi-Step Mole Conversions Multi-step mole conversions require the use of two or more conversion factors. Before trying multi-step conversions, there is one more type of conversion factor to be aware of:
For Example: How many atoms are there in 5 molecules of CuSO 4 5 H 2 O? How many Hydrogen atoms are there in 30 molecules of H 3 PO 4?
The most difficult type of conversions involve changing between various units requiring you to use several conversions. It s important for you to remember that the mole is central to ALL conversions between mass, particles, and volume. Each calculation goes from STARTING UNIT to MOLES to FINAL UNIT
Always remember:
For Example: What is the volume occupied by 50.0g of NH 3(g) at STP? What is the mass of 1.00x10 12 atoms of Cl? How many Oxygen atoms are contained in 75.0L of SO 3(g) at STP?
Problems involving density require you to recall: Where: d = density (in g/ml or kg/l_ m = mass (in g or kg) V = volume (in ml or L)
For Example: What is the volume occupied by 3.00 mol of ethanol, CH 3 CH 2 OH (l)? (d= 0.790 g/ml) How many moles of Hg (l) are contained in 100mL of Hg (l) (d = 13.6 g/ml) What is the density of O 2(g) at STP? A 2.50 L bulb contains 4.91g of a gas at STP. What is the molar mass of the gas? Al 2 O 3(s) has a density of 3.97 g/ml. How many atoms of Al are in 100mL of Al 2 O 3(s)?
Homework: Finish 4-1 Finish 10-2 (hand in) Multi-Step Mole Conversion W.S.