mole problems

MOLE PROBLEMS

CALCULATING MOLAR MASS

Round all masses to the 10ths place

CALCULATE THE MOLECULAR WEIGHT OF Fe2O3

Step One: two atoms of iron and three of oxygen

Step Two: Fe = 55.8 amu and O = 16.0 amu

Step Three: (2 x 55.8) + (3 x 16.0)

Step Four: 111.6 + 48.0 = 159.6 amu (grams) for one mole

CALCULATE THE MOLECULAR WEIGHT OF KClO4

Step One: one atom of potassium, one of chlorine, and four of oxygen.

Step Two: K = 39.1 amu, Cl = 35.5 amu, O = 16.0 amu

Step Three: (1 x 39.1) + (1 x 35.5) + (4 x 16.0)

Step Four: 39.1 + 35.5 + 64.0 = 138.6 amu (grams) for one mole

CALCULATE THE MOLECULAR WEIGHT OF (NH4)2S

Step One: two atoms of nitrogen, eight of hydrogen and one of sulfur

Step Two: N = 14.0 amu, H = 1.0 amu, S = 32.1 amu

Step Three: (2 x 14.0) + (8 x 1.0) + (1 x 32.1)

Step Four: 28 + 8.0 + 32.1 = 68.1 amu (grams) for one mole

CALCULATE THE MOLAR MASS OF Al(NO3)3

Calculate the molar mass of Al(NO3)3

Al(1 x 27.0) + N(3 x 14.0) + O(9 x 16.0) =

213.0 amu (grams) is the mass of one mole of aluminum nitrate.

YOU TRY THESE… 1. AlCl3 2. Ba(SCN)2 3. LiH 4. Ba(BrO3)2 5. AlBr3 6. HCl

ANSWERS 1. 133.5 g/mol 2. 253.5 g/mol 3. 7.9 g/mol 4. 393.1 g/mol 5. 266.7 g/mol 6. 36.5 g/mol

GRAMS TO MOLES

Round all final answers to 3 sig figs

CONVERT 25.0 GRAMS OF KMnO4 TO MOLES.

KMnO4 = 158.0 g / mole of KMnO4

Now let’s solve it

25.0 g x 1 mole = 0.158191804 moles 158.0 g

0.158 moles

CALCULATE HOW MANY MOLES ARE IN 17.0 GRAMS OF H2O2. H2O2 = 34.0 g / mole

Now, let’s solve it

17.0 g x 1 mol H2O2 = 0.5 moles 34.0 g

0.500 moles

YOU TRY THESE… Calculate the moles present in: 1) 2.00 grams of H2O 2) 75.57 grams of KBr 3) 100. grams of KClO4 4) 8.76 grams of NaOH 5) 0.750 grams of Na2CO3

Remember to round answers to 3 sig figs

ANSWERS 1) 0.111 moles 2) 0.635 moles 3) 0.722 moles 4) 0.219 moles 5) 0.00708 moles

MOLES TO GRAMS

Round all answers to 3 sig figs

CALCULATE HOW MANY GRAMS ARE IN 0.700 MOLES OF H2O2

H2O2 molar mass = 34.0 grams/mole

0.700 moles x 34.0 grams = 23.8 g 1 mole

23.8 grams

CONVERT 2.50 MOLES OF KClO3 TO GRAMS. KClO3 molecular mass = 122.6 g/mole

2.50 moles x 122.6 g = 306.5 grams 1 mole

307 grams

YOU TRY THESE… Calculate the grams present in: 1) 0.200 moles of H2S 2) 0.100 moles of KI 3) 1.500 moles of KClO 4) 0.750 moles of NaOH 5) 3.40 x 10-5 moles of Na2CO3

Remember to round answer to 3 sig figs

ANSWERS 1) 6.82 grams 2) 16.6 grams 3) 136 grams 4) 30.0 grams 5) 0.00360 grams

AVOGADRO'S NUMBER

6.02 x 1023 particles = 1 mole

Particles can mean atoms or molecules

ONE MOLE OF ANY SPECIFIED ENTITY CONTAINS 6.02 X 1023

OF THAT ENTITY

FOR EXAMPLE One mole of donuts contains 6.02 x 1023

donuts

One mole of H2O contains 6.02 x 1023 molecules

One mole of nails contains 6.02 x 1023 nails

One mole of Fe contains 6.02 x 1023 atoms

CONTINUED One mole of dogs contains 6.02 x 1023

dogs

One mole of electrons contains 6.02 x 1023 electrons

One mole of Chemistry students contains 6.02 x 1023 poor, suffering (I mean happy, joyful) high school students

0.450 MOLE OF Fe CONTAINS HOW MANY ATOMS? 0.450 mole Fe x 6.02 x 1023 atoms

1 mole

= 2.709x1023 atoms

Final, rounded answer: 2.71x1023 atoms

0.200 MOLE OF H2O CONTAINS HOW MANY MOLECULES? 0.200 mole x 6.02 x 1023 atoms

1 mole

= 1.204x1023 atoms

Final, rounded answer: 1.20x1023 atoms

YOU TRY THESE… 1) Calculate the number of molecules in

1.058 mole of H2O

2) Calculate the number of atoms in 0.750 mole of Fe

Remember to round to 3 sig figs

ANSWERS 1) 6.37x1023 molecules

2) 4.52x1023 atoms

TWO STEP PROBLEMS

Grams to particles OR particles to grams

Grams to atoms OR atoms to grams

If MOLE is not stated, it’s a two step problem

ROUND ALL ANSWERS TO 3 SIG FIGS

0.450 GRAM OF Fe CONTAINS HOW MANY ATOMS? 0.450 g x 1 mole Fe x 6.02x1023 atoms

= 55.8 g 1 mole

4.8548387x1021 atoms

Final answer = 4.85x1021 atoms

0.200 GRAM OF H2O CONTAINS HOW MANY MOLECULES? H2O molecular mass = 18.0 g

0.200 g x 1 mole H2O x 6.02x1023 molecules 18.0 g 1 mole

6.6888888889x1021 molecules

Final answer = 6.69x1021 molecules

YOU TRY THESE… 1) Calculate the number of molecules in

1.058 gram of H2O

2) Calculate the number of atoms in 0.750 gram of Fe

Remember to round answer to 3 sig figs

ANSWERS 1) 3.54x1022 molecules

2) 8.10x1021 atoms

THE END