Review Self-Quiz for Stoichiometry

1. Write the formula for a compound made from sodium and chlorine.

2. Write a chemical reaction for the following: Na reacts with chlorine, Cl2, to form sodium chloride.

3. Find the gram formula weight of Cl2.

4. Find the gram formula weight of NaCl

Na+ and Cl- → NaCl

2 Na + Cl2 → 2 NaCl

2 Cl = 35 x 2 = 70 g/moleNa = 23Cl = 35 58 g/mole

Basic Skills• Looking up the mass of elements in

the periodic table:1. Li2. Na3. Ca4. Cu5. Fe6. N7. O8. Cl9. S10.Br

6.941 amu or g/mole22.989 768 amu or g/mole

40.078 amu or g/mole63.546 amu or g/mole

55.847 amu or g/mole14.006 74 amu or g/mole

15.999 4 amu or g/mole35.452 7 amu or g/mole32.066 amu or g/mole

79.904 amu or g/mole

Basic Skills needed• Finding the gram

formula mass• separate the elements• look up the atomic mass

in the periodic table• figure out how many of

each atom are in the formula

• subtotal the masses of the elements

• Find the total mass of the compound.

ex: Al2(SO4)3

AlSO

273216

2312

x 2x 3x 12

= 54= 96= 192

= 342g mole

Stoichiometry (Chapter 9 Black Book)

• READ and UNDERLINE important parts of the problem.

• Write Balanced Formulas• + = -

• Write Balanced Equations• use coeficients to make atoms in reactants =

same atoms in products• Find moles of “known”• Find moles of “unknown”• Make sure the questions are answered.

Your Turn: Find the gfw for the following compounds:1. Ca (OH)2

2. NaOH3. CaCl24. MgF2

5. K2O6. Zn3(PO4)2

7. NaCl

answers:Ca(OH)2 = 74 g/moleNaOH = 40 g/moleCaCl2 = 110 g/moleMgF2 = 62 g/moleK2O = 94 g/moleZn3(PO4)2= 385 g/moleNaCl = 58 g/mole

What mass of NH4Cl can be produced from 7.87 grams of NH3. NH3 + HCl → NH4Cl

• Known: 7.87 g NH3

• Unknown: ? g NH4Cl

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Next, to convert the mass of NH3 to molesyou must find the gram formula weight (gfw)of ammonia, NH3

N = 14H = 13 x 3 =3

=14

------17 g/mole NH3

First, identify knowns and unknowns

What mass of NH4Cl can be produced from 7.87 grams of NH3. NH3 + HCl → NH4Cl

• Known: 7.87 g NH3 = ? mole• Unknown: ? g NH4Cl

_________________________________________________________________

Now that you know that NH3 has a gfw (gramformula weight of 17 g/mole, you convert grams ammonia to moles of ammonia.

------17 g/mole NH3

7.87 g NH3 = 0.463 moles NH3

What mass of NH4Cl can be produced from 7.87 grams of NH3. NH3 + HCl → NH4Cl

• Known: 7.87 g NH3 = 0.463 mole• Unknown: ? g NH4Cl = .463 mole

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Now that you know that we have 0.463 moleof NH3, we use the balanced chemical equation to determine that for every 1 moleof NH3 we make 1 mole of NH4Cl.

----------------------- = 0.463 moles NH4Cl1 mole NH4Cl

1 mole NH3

0.463 moles NH3

What mass of NH4Cl can be produced from 7.87 grams of NH3. NH3 + HCl → NH4Cl

• Known: 7.87 g NH3 = 0.463 mole• Unknown: ? g NH4Cl = 0.463 mole

_________________________________________________________________

Now that you know that we have 0.463 moleof NH4Cl, we find the gfw (gram formulaweight) of NH4Cl.

N = 14H = 1Cl = 35

4 x 4 = 4=35

=14

53 g/mole NH4Cl

What mass of NH4Cl can be produced from 7.87 grams of NH3. NH3 + HCl → NH4Cl

• Known: 7.87 g NH3 = 0.463 mole• Unknown: ? g NH4Cl = 0.463 mole

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Now that you know that we have 0.463 moleof NH4Cl, and the gfw (gram formulaweight) of NH4Cl is 53g/mole, we can calculate the number of grams ammonium chloride.

53 g/mole NH4Cl(0.463 mole NH4Cl) = 24.5 g NH4Cl

Try another:

• What mass of Mg(OH)2 will react with 1.20 grams of HCl.• Mg(OH)2 + 2 HCl -> MgCl2 + 2 H2O

1.20 g

(36 g/mole HCl)

0.033 moles HCl

21

0.017

Mg = 242O = 322H = 2 = 58g/mole

x 58g/mole

0.97 grams

Try another:

• What mass of Ca(OH)2 will react with 2.40 grams of HCl.• Ca(OH)2 + 2 HCl -> CaCl2 + 2 H2O

2.40 g

(36 g/mole HCl)

0.067 moles HCl

21

0.033

Ca = 402O = 322H = 2 = 74g/mole

x 74 g/mole

2.5 grams

Try another:

• What mass of CaCl2 will form when 2.40 grams of HCl react.• Ca(OH)2 + 2 HCl -> CaCl2 + 2 H2O

2.40 g

(36 g/mole HCl)

0.067 moles HCl

2 1

Ca = 402Cl = 70 = 110g/mole 0.033 moles CaCl2

x 110 g/mole CaCl2

3.7 g CaCl2?