Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
THE MOLE How Scientists Keep Track of Atoms • One way to measure how much substance available is to count the # of particles in that sample – However, atoms & molecules are extremely small – Also, the # of individual particles in even a small sample is very large – Therefore, counting the # of particles is not a practical measure of amount • To solve this problem, scientists developed the concept of the mole – It’s the “chemical counting unit” Counting by weighing • 1 Bean 5 grams • 5 beans 50 grams • HOW? Average Mass • Mass out 50 beans and find the average Multiple Stuff • I Bean 5 grams • 1 Mint 15 grams • Just as a dozen eggs equals 12 eggs, a mole = 602,000,000,000,000,000,000,000 – It is equal to that number no matter what kind of particles you’re talking about – It could be represent marbles, pencils, or bikes – usually deals with atoms and molecules • The word “mole” was introduced about 1896 by wilhelm oswald, who derived the term from the latin word moles meaning a “heap” or “pile.” • The mole, whose abbreviation is “mol”, is the SI base unit for measuring amount of a pure substance. • The mole is the chemist’s six-pack or dozen. Many objects in our everyday lives come in similar counting units. 1 dozen = 12 23 1 mole = 6.02x10 1 dozen eggs = 12 eggs 1 mole eggs = 6.02x1023 eggs 2 doz of atoms = 24 atoms 2 mols of atoms=1. 24 20x10 atoms • THE MOLE, AS A UNIT, IS ONLY USED TO COUNT VERY SMALL ITEMS – REPRESENTS A # OF ITEMS, SO, WE CAN KNOW EXACTLY HOW MANY ITEMS ARE IN 1 MOLE • THE EXPERIMENTALLY DETERMINED NUMBER A MOLE IS THE EQUIVALENT OF IS CALLED: AVOGADRO’S #= 23 or 6.02x10 REPRESENTATIVE PARTICLES • THE TERM REPRESENTATIVE PARTICLE REFERS TO THE SPECIES PRESENT IN A SUBSTANCE – USUALLY ATOMS – MOLECULES – OR FORMULA UNITS (IONS) • IT’S IMPORTANT TO NOTE THAT A DOZEN CUPS OF MARBLES CONTAINS MORE THAN A DOZEN MARBLES – SIMILARLY, A MOLE OF MOLECULES CONTAINS MORE THAN A MOLE OF ATOMS REPRESENTATIVE PARTICLES & MOLES ATOMIC NITROGEN ATOM N 6.02x1023 NITROGEN GAS MOLEC. N2 6.02x1023 WATER MOLEC. H 20 6.02x1023 CALCIUM ION ION Ca2+ 6.02x1023 CALCIUM FLUORIDE FORMULA UNIT CaF2 6.02x1023 HOW DO WE USE THE MOLE? • SINCE THE MOLE IS SUCH A HUGE NUMBER OF ITEMS, IT IS ONLY USED TO DESCRIBE THE AMOUNT OF THINGS THAT ARE VERY, VERY SMALL. • WE’D NEVER USE THE MOLE TO DESCRIBE MACROSCOPIC OR REAL WORLD OBJECTS. HOW BIG A NUMBER ARE WE TALKING? 1 mole = 23 6.02x10 • 6.02x1023 Watermelon seeds: would be found inside a melon slightly larger than the moon. • 6.02x1023donut holes: would cover the earth and be 5 miles deep. • 6.02x1023 grains of sand: would be cover miami beach 10 ft deep • 6.02x1023bloodcells: would be more than the total # of blood cells found in every human on earth USING THE MOLE IN CALCULATIONS #1 HOW MANY MOLES OF MAGNESIUM IS 1.25x1023 ATOMS OF MAGNESIUM? OUR UNIT EQUALITY TO DO THIS CONVERSION IS 1 mol Mg = 6.02x1023 atoms Mg THE DESIRED CONVERSION IS: ATOMS MOLES 1.25x1023 atoms Mg 1 mole Mg 6.02x1023atoms Mg = .208 mol Mg • NOW SUPPOSE YOU WANT TO DETERMINE HOW MANY ATOMS ARE IN A MOLE OF A COMPOUND – TO DO THIS YOU MUST KNOW HOW MANY ATOMS ARE IN A REPRESENTATIVE PARTICLE OF THE COMPOUND. • TO DETERMINE THE NUMBER OF ATOMS IN A PARTICLE REQUIRES KNOWING THE CHEMICAL FORMULA – FOR EXAMPLE, EACH MOLECULE OF CARBON DIOXIDE (CO2) IS COMPOSED OF 3 ATOMS • 1 MOLE OF CARBON DIOXIDE CONTAINS AVOGADRO’S NUMBER OF CARBON DIOXIDE MOLECULES. – THUS A MOLE OF CO2 CONTAINS THREE TIMES AVOGADRO’S NUMBER OF ATOMS • TO FIND THE # OF ATOMS IN A MOL OF A COMPND, – YOU 1ST DETERMINE THE # OF ATOMS IN A REPRESENTATIVE PARTICLE OF THAT COMPND – AND THEN MULTIPLY THAT # OF ATOMS BY AVOGADRO’S # USING THE MOLE IN CALCULATIONS #2 HOW MANY ATOMS ARE IN 2.12 mols OF PROPANE (C3H8)? UNIT EQUALITIES ARE 1 molecule C3H8 = 11 atoms C3H8 THE DESIRED CONVERSIONS ARE: MOLES MOLECULES ATOMS 23 molecules 6.02x10 2.12 moles C3 H8 C3 H8 = 1 mole C3H8 1.276x1024 11 atoms C3H8 molecules C3 H8 1 molecule C3H8 25 1.40x10 atoms C 3 H8 = ATOMIC MASSES • WHAT IS AN ATOM’S MASS? – IF MEASURED IN GRAMS, THE MASSES OF ATOMS WOULD BE TOO SMALL TO WORK WITH – THEREFORE, INSTEAD OF USING THE ACTUAL MASS OF A CARBON ATOM IN GRAMS, CHEMISTS USE RELATIVE ATOMIC MASSES ATOMIC MASS UNITS • IN DETERMINING RELATIVE MASSES, ONE ATOM IS ARBITRARILY CHOSEN AS THE STANDARD • THE MASS OF ALL THE OTHER ATOMS ARE THEN EXPRESSED IN TO THIS STANDARD • RELATION FOR THE RELATIVE VALUE MASS OF AN ATOM CHEMISTS AGREED UPON THE CARBON-12 ATOM ATOMIC MASS UNITS • A SINGLE CARBON-12 ATOM WAS ASSIGNED THE VALUE OF 12 ATOMIC MASS UNITS (AMU). • THEREFORE, 1 ATOMIC MASS UNIT IS EXACTLY 1/12 OF THE MASS OF A CARBON-12 ATOM – HYDROGEN THEN WEIGHS 1 AMU – HELIUM WEIGHS 4 AMUS • AMU’S GAVE SCIENTISTS A UNIT TO WORK WITH, BUT IT STILL DESCRIBED THE MASS IN TERMS OF INDIVIDUAL ATOMS • SCIENTISTS MUST FIGURE OUT A WAY TO WORK WITH A COLLECTION OF PARTICLES THAT AREN’T HANDLED INDIVIDUALLY • AND THAT CAN STILL BE THOUGHT OF IN TERMS OF A RELATIVE (OR COMPARED) MASS – AN AVERAGE C ATOM WITH AN ATOMIC MASS OF 12.0 amu IS 12 TIMES HEAVIER THAN AN AVERAGE H ATOM WITH AN ATOMIC MASS OF 1.0 amu – THEREFORE, 100 C ATOMS ARE 12 TIMES HEAVIER THAN 100 H ATOMS • ANY NUMBER OF C ATOMS IS 12 TIMES HEAVIER THAN THE SAME # OF H ATOMS – SO, IF WE HAD IF WE HAD A PILE OF CARBON ATOMS THAT WEIGHED 12g AND A PILE OF HYDROGEN ATOMS THAT WEIGHED 1g, – BOTH PILES SHOULD CONTAIN THE SAME NUMBER OF ATOMS • THE GRAM ATOMIC MASSES OF ANY 2 ELEMENTS (SINCE THEY ARE RELATIVE TO CARBON) MUST CONTAIN THE SAME NUMBER OF ATOMS • A PILE OF ANY ATOM THAT CORRESPONDS TO ITS AVERAGE ATOMIC MASS FROM THE PT CONTAINS EXACTLY 6.02x1023 ATOMS OF THAT ELEMENT. – ALSO CALLED A MOLE CARBON ATOMS 12 HYDROGEN ATOMS 1 MASS RATIO • WHAT THIS ALLOWS US TO DO IS TO USE THE MASS OFF OF THE PERIODIC TABLE TO REPRESENT HOW MUCH 1 MOLE OF THAT ELEMENT WEIGHS – 1 MOLE OF CARBON ATOMS WEIGH 12.01 g – 1 MOLE OF HYDROGEN ATOMS WEIGH 1.008 g – 1 MOLE OF TUNGSTEN ATOMS WEIGH 183.8; ETC. • THIS NEW VERSION OF MASS FROM THE PERIODIC TABLE IS CALLED THE GRAM MOLAR MASS, OR MOLAR MASS. – MOLAR MASS = MASS OF 1 MOLE OF ATOMS/MOLECULES/OR FORMULA UNITS IN GRAMS – SYMBOL = MM – UNITS = GRAMS/MOLE • SO HOW DO WE FIGURE OUT THE MASS OF A MOLE OF A COMPOUND RATHER THAN JUST 1 ELEMENT? • TO ANSWER THAT QUESTION YOU MUST HAVE THE FORMULA OF THE COMPOUND. – THE FORMULA OF A COMPND TELLS YOU HOW MANY ATOMS OF EACH ELEMENT COMBINE TO MAKE THE REPRESENTATIVE PARTICLE OF THAT COMPND. Formula Model Formula Model • YOU CAN CALCULATE THE MASS OF A MOLECULE OF SO3 BY ADDING THE MOLAR MASSES OF THE ATOMS THAT MAKE UP THE MOLECULE – FROM THE PERIODIC TABLE, THE MASS OF SULFUR IS 32.1g/mol. – THE MASS OF THREE ATOMS OF OXYGEN IS 3 TIMES THE MOLAR MASS OF A SINGLE OXYGEN ATOMS, WHICH IS (3)(16g/mol) OR 48g/mol • THE TOTAL MASS OF EACH OF THE ATOMS IN 1 MOLECULE OF SO3 = 32.1g/mol + 48 g/mol = 80.1 g/mol CALCULATING MOLAR MASSES USING CHEMICAL FORMULAS MM of C6H12O6: (6C’S)(12g/mol)= (12H’S)(1 g/mol)= (6O’S)(16 g/mol)= 72g/mol 12g/mol 96g/mol 180g/mol IF WE HAD 1 MOLE OF THE COMPND C6H12O6 OR 6.02X1023 MOLECULES OF THE COMPND – IT WOULD WEIGH 180 grams • WE CAN USE THE MOLAR MASS OF AN ELEMENT OR COMPOUND AS A CONVERSION FACTOR TO CONVERT BETWEEN GRAMS AND MOLES OF A SUBSTANCE. – THE UNIT EQUALITY IS 1 MOLE = __ MM OF THE SUBSTANCE USING THE MOLE IN CALCULATIONS #3 HOW MANY GRAMS ARE IN 9.45 mol OF DINITROGEN TRIOXIDE (N2O3) UNIT EQUALITY TO USE THE DESIRED CONVERSION IS: MOLES GRAMS 9.45 mol N 2O 3 76 grams N2O3 1 mole N2O3 = 718 g N2O3 USING THE MOLE IN CALCULATIONS #4 FIND THE NUMBER OF MOLES OF 92.2g OF IRON (III) OXIDE (Fe2O3) UNIT EQUALITY TO USE THE DESIRED CONVERSION IS: GRAMS MOLES 92.2 g Fe2O3 1 mole Fe2O3 159.6 g Fe2O3 = 0.578 mol Fe2O3 VOLUME AND THE MOLE • UNDER THE SAME CONDITIONS, EQUAL VOLUMES OF GASES CONTAIN THE SAME NUMBERS OF PARTICLES. – OR 1 MOLE OF A GAS WILL OCCUPY THE SAME VOLUME AS 1 MOLE OF ANY OTHER GAS UNDER THE SAME CONDITIONS. • IT’S KNOWN AS THE MOLAR VOLUME OF A GAS MOLAR VOLUME 1 MOLE OF ANY GAS AT STP (0°C and 1 atm) HAS A VOLUME OF: 1 mole = 22.4 L USING THE MOLE IN CALCULATIONS #5 DETERMINE THE VOLUME, IN LITERS, OF 0.60 molSO2 GAS AT STP. UNIT EQUALITY TO USE THE DESIRED CONVERSION IS: MOLES LITERS 0.60 moles SO2 22.4 L SO2 1 mole SO2 = 13 L SO2 AVOGADRO’S NUMBER MASS (in grams) Volume (of gas at STP) MOLAR MASS MOLE MOLAR VOLUME GOOD EXAMPLE PROBLEM! IF YOU HAVE A 35.67g PIECE OF CHROMIUM METAL ON YOUR CAR, HOW MANY ATOMS OF CHROMIUM DO YOU HAVE? • YOU ARE GIVEN MASS AND ASKED FOR NUMBER OF PARTICLES • LET’S GET SOME STRATEGY WE ARE GIVEN MASS WE ARE ASKED FOR ATOMS WE ARE GIVEN MASS WE ARE ASKED FOR ATOMS WE ARE GIVEN MASS •IT’S GOING TO TAKE US 2 STEPS, WE JUST FOLLOW THE ARROWS • THE FIRST STEP IS TO CONVERT OUR GIVEN GRAMS INTO MOLES • TO DO THIS WE USE THE MOLAR MASS (MM) OF CHROMIUM WHICH ON THE PT IS 52g/mol 35.67g Cr 1 mole Cr 52 g Cr = .686 mole Cr • THE SECOND STEP WE ARE GOING TO TAKE OUR NEWLY CALCULATED MOLES OF Cr AND CONVERT IT TO THE NUMBER OF ATOMS OF Cr • WE HAVE TO REMEMBER THAT IF WE HAD 1 MOLE OF Cr ATOMS WE 23 .686 mole Cr 6.02x1023 atoms Cr 1 mole Cr = 4.130x1023 atoms Cr