Mole Relationships in Chemistry

Transcription

1 Mole Relationships in Chemistry

2 The Mole Concept and Atomic Masses The mole concept and molar mass is historically based on two laws from Joseph-Louis Proust in 1797 The Law of Definite Proportions This states that compounds always have a definite proportion of the elements that make it up These proportions can be expressed as ratios of atoms, equivalent mass values, percentage by mass or volumes of gaseous elements Example - Water always contains 2 H atoms for every atom, which is 2 g H for every 16 g or 11.1% H and 88.9% by mass The Law of Multiple Proportions These laws are based on mass John Dalton s atomic theory used these laws to formulate his atomic theory His atomic theory is based on counts of atoms The relationship between mass and count is done through the mole

3 Deriving The Mole from Masses The number of atoms in grams of 12 C can be calculated: ne atom 12 C = u = 12 x (1.661 x g) = x g / atom # atoms = g (1 atom / x g) = x atoms The number of atoms of any element needed to equal its atomic mass in grams will always be x atoms Called the mole

4 Why is the Mole So Important? 1 mole of any element = x atoms Using moles gives us a practical AND measurable unit! Atoms, ions and molecules are too small to directly measure in atomic mass units So by using the mole, we can relate atoms, ions and molecules, using an easy to measure unit -the gram!

5 The Mole

6 Molar Mass Atoms come in different sizes and masses A mole of atoms of one type would have a different mass than a mole of atoms of another type H grams / mol grams / mol Mo grams / mol Pb grams / mol We rely on a straight forward system to relate mass and moles which you learned in first year chemistry!

7 Using Molar Mass to Calculate % Composition (Mass %) We can use the molar mass of a compound to determine the relative masses of the elements in a compound btained by comparing the MASS F EACH ELEMENT present in 1 mole of the compound to the TTAL MASS of 1 mole of the compound A pure compound should show the same percent mass of each element consistently So given a formula, you should be able to figure out the percent mass of each element Mass % = mass of element in 1 mole of compound 100 mass of 1 mole of compound

8 Elemental Analysis to Determine Mass Percent of a Compound A sample is burned, completely converting it to C 2 and H 2 Each is collected and measured as a weight gain By adding other traps elements like oxygen, nitrogen, sulfur and halogens can also be determined 2 furnace C 2 trap H 2 trap sample

9 Elemental Analysis Example A compound known to contain only carbon, hydrogen and nitrogen is examined by elemental analysis. The following information is obtained. riginal sample mass Mass of C 2 collected Mass of H 2 collected = g = g = g Determine the % of each element in the compound

10 Elemental Analysis Mass of carbon g C g C g C 2 Mass of hydrogen g H g H 2 Mass of nitrogen g H 2 = g C = g H g sample g C g H = g N

11 Elemental Analysis Since we know the total mass of the original sample, we can calculate the % of each element g % C = x 100% = % g % H = g x 100% = % g % N = g x 100% = % g

12 Using % Composition to Determine Chemical Formulas Based on the relative masses of the elements in a compound, we can directly calculate an empirical chemical formula The lowest whole number ratio of elements in a compound CH 2 We need additional information to determine the molecular formula The actual ratio of elements in a compound C 2 H 4 C 3 H 6 There are instances where empirical and molecular formula can be the same! H 2

13 Calculating Empirical Formulas from % Composition Pretend that you have a 100 gram sample of the compound Change the % to grams Convert the grams to moles for each element Write the number of each element as a subscript in a chemical formula Keep each number as a decimal at this point! Divide each subscript by the smallest number Multiply the result by some integer to get rid of any fractions May not be necessary

14 How to Convert between Empirical Formulas and Molecular Formulas Since the empirical formula is the lowest ratio, the actual molecule would weigh more Molecular formula can always be obtained by multiplying by some whole number To do so, divide the actual molecular molar mass (usually given in the problem) by the mass of 1 mole of the empirical formula Gives whole number you MUST multiply the empirical formula by to get the molecular formula x Molecular molar mass Empirical molar mass

15 Using the Mole to Calculate Concentration of Solutions Concentration is commonly expressed in terms of molarity Defined mathematically as: M = moles of solute volume of solution (L) = n solute L solution M is read as molar Molarity recognizes that compounds have different molar masses A 1-molar solution of sucrose contains the same number of molecules as 1-molar solution of ethanol

16 ther Methods of Expressing Concentration When making different solutions with a specific molarity, the number of milliliters of solvent needed to prepare 1 liter of solution will vary Sometimes it is necessary to know the exact proportions of solute to solvent that are in a particular solution Various methods have been devised to express these proportions

17 Molality moles solute kilograms of solvent Molality (m) = = mol kg Recognizes that the ratio between moles of solute and kg of solvent can vary A 1-molal solution of sucrose contains the same number of molecules as 1-molal ethanol