Skills Worksheet Problem Solving

1 Copyright by Holt, Rinehart and Winston. All rights ChemFile: Problem - Solving Workbook48 Mole ConceptMole ConceptSuppose you want to carry out a reaction that requires combining one atom ofiron with one atom of sulfur. How much iron should you use? How much sulfur?When you look around the lab, there is no device that can count numbers ofatoms. Besides, the merest speck ( g) of iron contains over a billion billionatoms. The same is true of , you do have a way to relate mass and numbers of atoms. One ironatom has a mass of amu, and g of iron contains 137 1023atoms of iron. Likewise, g of sulfur contains 137 1023atoms of sul-fur.

2 Knowing this, you can measure out g of iron and g of sulfur andbe pretty certain that you have the same number of atoms of each. The number 137 1023is called Avogadro s number. For most purposesit is rounded off to 1023. Because this is an awkward number to writeover and over again, chemists refer to it as a mole(abbreviated mol). 1023objects is called a mole, just as you call 12 objects a again at how these quantities are g of iron 1023iron atoms 1 mol of g of sulfur 1023sulfur atoms 1 mol of sulfurGeneral Plan for Converting Mass, Amount,and Numbers of ParticlesUse Avogadro'snumber for ofsubstanceAmount ofsubstancein molesNumber of atoms,molecules, or formulaunits of substanceConvert usingthe molar mass ofthe SolvingSkills WorksheetCopyright by Holt, Rinehart and Winston.

3 All rights ChemFile: Problem - Solving Workbook49 Mole ConceptNameClassDateProblem Solving continuedPROBLEMS INVOLVING ATOMS AND ELEMENTSS ample Problem 1A chemist has a jar containing g of iron filings. How many moles ofiron does the jar contain?SolutionANALYZEWhat is given in the Problem ?mass of iron in gramsWhat are you asked to find?amount of iron in molesPLANWhat step is needed to convert from grams of Fe to number of moles of Fe?The molar mass of iron can be used to convert mass of iron to amount of iron the units correct?Yes; the answer has the correct units of moles of mol g Fe mol g Fe multiply by the inversemolar mass of FeMass of Fe in g1 Amount of Fe in mol21molar mass Fegiven1 mol Fe mol Feg Fe g FeItemsDataMass of gMolar mass of iron* g/molAmount of iron?

4 Mol* determined from the periodic tableCopyright by Holt, Rinehart and Winston. All rights ChemFile: Problem - Solving Workbook50 Mole ConceptNameClassDateProblem Solving continuedIs the number of significant figures correct?Yes; the number of significant figures is correct because there are four signifi-cant figures in the given value of g the answer reasonable?Yes; g Fe is about seven times the molar mass. Therefore, the samplecontains about 7 the number of moles in each of the following g of aluminum ans: mol g of silicon ans: mol g of sulfur ans: 10 3mol g of zinc ans: mol ZnCopyright by Holt, Rinehart and Winston.

5 All rights ChemFile: Problem - Solving Workbook51 Mole ConceptNameClassDateProblem Solving continuedSample Problem 2A student needs mol of zinc for a reaction. What mass of zinc ingrams should the student obtain?SolutionANALYZEWhat is given in the Problem ?amount of zinc needed in molesWhat are you asked to find?mass of zinc in gramsPLANWhat step is needed to convert from moles of Zn to grams of Zn?The molar mass of zinc can be used to convert amount of zinc to mass of the units correct?Yes; the answer has the correct units of grams of the number of significant figures correct?Yes; the number of significant figures is correct because there are three signifi-cant figures in the given value of mol the answer reasonable?

6 Yes; mol is about 1/3 mol. g is about 1/3 the molar mass of g Zn1 mol Zn g mol Zn multiply by themolar mass of ZnAmount of Zn in mol2 Mass of Zn in mol1molar mass g Zn g Znmol Zn 1 mol ZnItemsDataAmount of molMolar mass of g/molMass of zinc? gCopyright by Holt, Rinehart and Winston. All rights ChemFile: Problem - Solving Workbook52 Mole ConceptNameClassDateProblem Solving the mass of each of the following mol sodium ans: g mol copper ans: 921 g mol mercury ans: g 10 3mol magnesium ans: MgCopyright by Holt, Rinehart and Winston. All rights ChemFile: Problem - Solving Workbook53 Mole ConceptNameClassDateProblem Solving continuedSample Problem 3 How many moles of lithium are there in 1024lithium atoms?

7 SolutionANALYZEWhat is given in the Problem ?number of lithium atomsWhat are you asked to find?amount of lithium in molesPLANWhat step is needed to convert from number of atoms of Li to moles of Li?Avogadro s number is the number of atoms per mole of lithium and can be usedto calculate the number of moles from the number of the units correct?Yes; the answer has the correct units of moles of the number of significant figures correct?Yes; four significant figures is the answer reasonable?Yes; 1024is approximately twice Avogadro s number. Therefore, it isreasonable that this number of atoms would equal about 2 mol.

8 1 mol 1023 atoms Li mol 1024 atoms Li multiply by the inverse ofAvogadro's numberNumber of Li atoms3 Amount of Li in mol21 Avogadro's numbergiven1 mol Li mol Liatoms Li 1023 atoms LiItemsDataNumber of lithium 1024atomsAvogadro s number the 1023atoms/molnumber of atoms per moleAmount of lithium? molCopyright by Holt, Rinehart and Winston. All rights ChemFile: Problem - Solving Workbook54 Mole ConceptNameClassDateProblem Solving the amount in moles in each of the following 1023atoms of rubidium ans: mol 1022atoms of krypton ans: mol 700 000 000 atoms of lead ans: 10 15mol 1025atoms of vanadium ans: mol VCopyright by Holt, Rinehart and Winston.

9 All rights ChemFile: Problem - Solving Workbook55 Mole ConceptNameClassDateProblem Solving continuedCONVERTING THE AMOUNT OF AN ELEMENT IN MOLES TO THE NUMBER OF ATOMSIn Sample Problem 3, you were asked to determine the number of moles in 1024atoms of lithium. Had you been given the amount in moles andasked to calculate the number of atoms, you would have simply multiplied byAvogadro s number. Steps 2 and 3 of the plan for Solving Sample Problem 3 wouldhave been the number of atoms in each of the following mol bismuth ans: 1023atoms mol manganese ans: 1024atoms 000 2 mol helium ans: 1 1017atoms mol strontium ans: 1025atoms SrCopyright by Holt, Rinehart and Winston.

10 All rights ChemFile: Problem - Solving Workbook56 Mole ConceptNameClassDateProblem Solving continuedSample Problem 4 How many boron atoms are there in g of boron ?SolutionANALYZEWhat is given in the Problem ?mass of boron in gramsWhat are you asked to find?number of boron atomsPLANWhat steps are needed to convert from grams of B to number of atoms of B?First, you must convert the mass of boron to moles of boron by using the molarmass of boron . Then you can use Avogadro s number to convert amount in molesto number of atoms of the units correct?Yes; the answer has the correct units of atoms of the number of significant figures correct?