Created by Laura Peck

Text: Silberberg – Chemistry 4^th edition

• Percent to grams
• Grams to moles
• Divide by small
• Multiply 'til whole.

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• 1. How many grams of Ag are in 0.0342 mol of Ag?
• Ag(g) = 0.0342 mol Ag 107.9g Ag = 3.69g Ag
• 1 mol Ag
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• 2. How many Fe atoms are in 95.8 g of Fe?
• Mol Fe = 95.8g Fe 1 mol Fe = 1.72 mol Fe
• 55.85g Fe
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• #atoms Fe = 1.72 mol Fe 6.022x10²³ atoms Fe
• 1 mol Fe
• = 10.4x10²³ atoms Fe = 1.04x10²⁴ atoms Fe
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• 1. How many moles of carbon are in 0.315g of graphite?

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• 2. What is the mass in grams of 3.22x10²⁰ Mn atoms, the number found in bones?

Moles C =

0.315g C

1 mol C

12.01 gC

= 0.026 mol C

Mass (g) Mn =

3.22x10²⁰ Mn

1 mol Mn

3.022x10²³

1 mol Mn

54.04 g Mn

= 0.0576 g Mn

• 1. How many formula units are in 41.6g of ammonium carbonate?

Formula is (NH₄)₂CO₃ so calculate Molar Mass

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• Convert grams to moles:

41.6g (NH₄)₂CO₃

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• Convert moles to formula units:

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Molar Mass = 96.09 g/mol

96.09g (NH₄)₂CO₃

1 mol (NH₄)₂CO₃

= 0.433 mol (NH₄)₂CO₃

0.433 mol (NH₄)₂CO₃

1 mol (NH₄)₂CO₃

6.022x10²³ f.u. (NH₄)₂CO₃

= 2.61x10²³ f.u. (NH₄)₂CO₃

• Tetraphosphorus decaoxide reacts with water to form phosphoric acid, a major industrial acid.
• 1. what is the mass (g) of 4.65x10²² molecules of tetraphosphorus decaoxide?
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• 2. How many P atoms are present in this sample?
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4.65x10²² P₄O₁₀

3.022x10²³ P₄O₁₀

1 mol P₄O₁₀

1 mol P₄O₁₀

283. 885g P₄O₁₀

= 21.921g P₄O₁₀

4.65x10²² P₄O₁₀

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= (4.65x10²² P)

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X 4 =

1.86x10²³ P

• In mammals, lactose is broken down to glucose (C₆H₁₂O₆), the key nutrient for generating chemical potential energy.

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Molar Mass = (6x12.011)+(12x1.008)+(6x15.99)

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(72.066 C)+(12.096 H)+(95.994 O) = = 180.156 g/mol C₆H₁₂O₆

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Mass % = M_individual/M_total x 100

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Mass C = 72.066/180.156 x100 = 40%C

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Mass H = 12.096/180.156 x100 = 6.741%H

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Mass O = 95.994/180.156 x100 = 53.29%O

• Ammonium nitrate is a common fertilizer. Agronomists base the effectiveness of fertilizer on their nitrogen content.
• 1. Calculate the mass % of N in ammonium nitrate:
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• 2. How many grams of N are in 35.8kg of ammonium nitrate?

(NH₄)(NO₃) =

(2 x14.007 N)

+ (4x1.008 H)

+ (3x15.999 O)

(28.014 N) +

(4.032 H) +

(47.997 O)

= 80.043 g/mol (NH₄)(NO₃

%N =

28.014/80.043 x100 =

25% N

35.8 kg (NH₄)(NO₃)

1 kg (NH₄)(NO₃

1,000 g (NH₄)(NO₃

80.043 g (NH₄)(NO₃

28.014 g N

= 12,529.5 g N or 1.25x10⁴ g N

• Elemental analysis of a sample of an ionic compound showed 2.82g Na, 4.35g Cl, and 7.83g O. What is the empirical formula and name of the compound?

Mole Na = 2.82g Na 1 mol Na = 0.123mol Na

22.99g Na

Mole Cl = 4.35g Cl 1 mol Cl = 0.123mol Cl

35.45g Cl

Mole O = 7.83g O 1 mol O = 0.489mol O

15.999 g O

Preliminary formula: Na_0.123Cl_0.123O_0.489

Divide all subscripts by smallest: 0.123/0.123 = 1.00;0.489/0.123 = 3.98

Na_1.00Cl_1.00O_3.98 Round to nearest whole # Na₁Cl₁O₄

Empirical Formula is NaClO₄ the name is Sodium perchlorate

• An unknown metal M reacts with sulfur to form a compound with the formula M₂S₃. If 3.12g of M reacts with 2.88g of S, what are the names of M and M₂S₃?

Mols S =

2.88g S

1 mol S

32.07g S

= 0.0898 mol S

Mols M =

0.0898 mol S

2 mol M

3 mol S

= 0.0599 mol M

Molar mass M =

3.12 g M

0.0599 mol M

= 52.1 g/mol

Look on the periodic table and find the best match for M.

Best match for M is chromium, so M₂S₃ is chromium (III) sulfide

• During physical activity, lactic acid (M = 90.08 g/mol) forms in muscle tissue and is responsible for muscle soreness. Elemental analysis shows that this compound contains 40%C, 6.71%H, and 53.3%O.
• 1. Determine the empirical formula of lactic acid:
• Express all % mass as grams. So 40.0gC, 6.71gH, 53.3gO
• Find moles: C = 40.0g C 1mol C = 3.33 mol C
• 12.01g C
• Similarly we have 6.66 mol H and 3.33 mol O
• C_3.33H_6.66O_3.33 C_1.00H_2.00O_1.00 so Empirical formula: CH₂O
• 3.3
• 2. Determine the molecular formula:
• M lactic acid = 90.08 g/mol = 3.0 Multiply (CH₂O)3 = C₃H₆O₃
• M empirical f. = 30.03 g/mol

• One of the most widespread environmental carcinogens in benzo[a]pyrene (M = 252.30 g/mol). It is found in coal dust, in cigarette smoke and even in charcoal-grilled meat. Analysis of this hydrocarbon shows 95.21%C and 4.79%H. What is the molecular formula of benzo[a]pyrene?

List % as grams; so 95.21g C and 4.79g H

Moles C =

95.21g C

1 mol C

12.01g C

= 7.928 mol C

Moles H =

4.79g H

1 mol H

1.0079g H

= 4.75 mol H

C_7.928H_4.75

4.75

= C_1.67H_1.00

= E.F. C₅H₃

Whole # multiple:

252.30 g/mol

63.07 g/mol

= 4

Molecular formula = 4(C₅H₃) = C₂₀H₁₂

Find lowest whole #