How many moles are in 55.4g of Aluminum Sulfate? Once you know the molar mass of a compound, you can use that mass to determine the amount of moles in a gram amount of the substance or conversely, you can calculate the number of grams in a mole amount of the substance. So here is the math using the atomic masses given in the periodic table: 2(26.98 g/mol) + 3( 32.07 g/mol) + 12(16.00 g/mol) = 342.17 g/molĢ) Conversion from Grams to Moles or Moles to Grams For Aluminum Sulfate the totals are 2 Al, 3 S and 12 O. You use the periodic table as a reference for these masses and total them up:įor example, if we want to total the molar mass of Aluminum Sulfate Al 2(SO 4) 3, we need to determine the number and mass of each element in the compound. The molar mass of a compound also often called the molecular weight while on Earth, is simply the sum of all the individual element masses in the compound. I am going to review a few of them now and then you will see as we progress through later topics that moles are essential to those calculations as well.ġ) Calculation of the Molecular Weight or Molar Mass of a compound There are plethora of calculations that can be conducted using the unit of moles as the intermediate. So a single carbon atom weighs 12 amu while a mole of carbon atoms weighs 12.01g/mol. So in reality, 1 AMU = 1.66 x 10 -24g and while a carbon atom weighs 12 AMU, the mass shown in the periodic table is 12 atoms x 1.66 x 10 -24g x 6.022 x 10 23 atoms/ mol= 12 g/mol And that is correct, the mass shown in the periodic table is actually the mass of 6.022 x 10 23 carbon atoms or a mole of carbon atoms. This means that 1 AMU ≈ 1g right? and thus Carbon has a mass of 12amu, correct? But wait, this could not be the mass of a single carbon atom right? They are really, really tiny. Let's start the relationship discussion with the relationship between the mole and the AMU.Īn AMU is 1/12 the mass of a Carbon-12 atom which according to the periodic table weighs ~12g. Reactions are balanced based on the number of moles of each element in the reaction, solution concentrations are very often described in terms of moles per liter or moles per kg of solvent and we have already seen that the molecules or atoms of an element are reported as moles of the substance rather than the individual count of their particles. The reason the mole is so important is because we use the mole as the unit for most of the relationships in chemistry. So if we had exactly 18.014g of water we would have 1 mole of water. If we total up the gram amounts of each element in the water molecule = 15.998g/mol + 2(1.008g/mol) we get the molar mass of water = 18.014g/mol. The mass of oxygen equal to one mole of oxygen is 15.998 grams and the mass of one mole of hydrogen is 1.008 g. One mole of Water is composed of 1 mole of Oxygen and two moles of Hydrogen. To use our old friend water as an example: The Molar mass or Molecular Weight (interchangeable terms so long as we are on Earth) of a substance is the total of all the individual masses of the elements it contains. Moles of a Substance and the Molecular Weight We use the stoichiometry (fancy way of saying mole ratios in an equation) to make predictions about how much product will be made or reactant needed if we know one mole amount in a reaction. What can we do with moles? We use the unit to make calculations based on balanced chemical equations. This is why we state the atomic and molecular masses in units of grams per mole or g/mol. Lithium for instance has an atomic mass of 6.941 grams and this is equal to one mole of lithium. Using carbon as a reference, the atomic weights you see in the periodic table are also equal to one mole of those substances: This means that the atomic mass or atomic weight (12 grams) of carbon is equal to exactly 1 mole of carbon. Why use 12 grams? This is the theoretical atomic mass of the Carbon-12 isotope (6 protons and 6 neutrons).
The value given 6.022 x 10 23 is called Avagadro's number for the scientist that found the number of atoms in 12 grams of carbon 12. We use the mole (mol) to represent the amount of substances in chemistry because the numbers of atoms and molecules in each substance is so large. So if you had a mole of donuts you would have 6.022 x 10 23 donuts and a serious stomach ache.
So the mole is the title used for the amount 6.022 x 10 23 much the same way the word "dozen" is used for the amount 12. The MOLE (mol) is a unit of measurement that is the amount of a pure substance containing the same number of chemical units (atoms, molecules etc.) as there are atoms in exactly 12 grams of carbon-12 (i.e., 6.022 X 10 23). In this lecture we cover the Mole and Avagadro's Number as well as the calculations for Molar Mass and conversions using moles. The content that follows is the substance of lecture 8.
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