13

Chemical Math Formula Writing

For binary compounds, “criss-cross” the oxidation states, drop the signs and simplify. Change the

ending of the 2nd element to –ide.

Na +1

O-2

Ca +2

N -3

Ca3N2

-2

calcium nitride

Na2O -1

sodium oxide +1

+2

+3

+4

For compounds involving polyatomic ions,

Ba +2

NO3 -1

Ba +2

NO2 -1

Ba (NO3)2 Ba (NO2) 2

barium nitrate barium nitrite

Criss-cross and use parentheses when there’s more than one polyatomic.

For metals with multiple oxidation states, use a Roman Numeral in the name of the compound to

indicate the oxidation state.

Iron (II) Oxide Iron (III) Oxide Mn +2 II Mn

+3 III

Fe +2

O-2

Fe +3

O-2

Mn +4 IV

Mn +7 VII

FeO Fe2O3 Going backwards, “Cross-Criss”; remember: the first element is always positive.

Cr2O3 → Cr2 O3 → Cr +3

& O -2

→ chromium (III) oxide

Finding the total number of atoms per unit of compound:

Given: Al(ClO3)3 Al(ClO3)3 total # atoms = 1 + 3 + 9 = 13

Cl3 O9

For hydrates, the dot means add, don’t multiply. Given: CuSO4 • 5 H2O total # O atoms = 4 + 5 = 9

H10 O5

NOTE: when asked to find “moles” of atoms, the same procedure would apply; instead of 9 oxygen atoms, the answer would be

9 moles oxygen atoms.

For

nonmetals

with

multiple

oxidation

states,

choose the

top one.

The Roman

Numeral is the

actual oxidation

state of the element,

not the order in

which it appears.

14 % Composition by Mass

H2O = 2(1) + 16 = 18 amu % H = 2/18 x 100 = 11.1%

% O = 16/18 x 100 = 88.9%

For hydrates, REMEMBER: “The legal age for drinking water is 18.”

CuSO4 • 5 H2O

64 + 32 + 4(16) + 5(18) = 250 % H2O = 5(18) / 250 x 100 = 36%

Molecular Formula Empirical Formula Given: Empirical Formula = CH and Formula Mass = 78

- actual no. atoms - simplest ratio of atoms Find: Molecular Formula

H2O2 HO 1st ) CH 2nd ) 78/13 = 6 3rd ) 6(CH) = C6H6

N2O4 NO2 12 + 1 = 13

NH3 NH3

Given: EF = CH2 and FM = 42, Find: the MF.

What is the mass of 2.5 mol NaOH?

1) NaOH 2) 2.5 = x 3) x = 2.5 (40) = 100 g

40

23 + 16 + 1 = 40 g/mole

How many moles are present in 160 grams of NaOH?

1) Same as above. 2) x = 160 3) x = 4 mol

40

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Balancing Chemical Equations

Why? Conservation of mass (atoms); REMEMBER: the coefficient of “1” is assumed.

___H2 + ___O2 ___ H2O Sum of Coefficients = ____

___N2 + ___ H2 ___NH3 Sum of Coefficients = ____

____Cu + ____AgNO3 ____Cu(NO3)2 + ____Ag Sum of Coefficients = ____

mass part x 100

mass whole

number of moles = given mass

gram formula mass

15

Stoichiometry - using the balanced chemical equation to find quantities.

Given: 4 Fe + 3 O2 2 Fe2O3

Question: How many moles of O2 are needed to produce 4 moles of Fe2O3?

Remember: the coefficients represent moles.

3 = 2 2x = 12 x = 12/2 = 6 moles

x 4

Types of Reactions

A + B AB synthesis A + BC AC + B single replacement

AB A + B decomposition AB + CD AD + CB double replacement

Significant Figures 1st) All non-zero digits are significant.

2nd) The position of the zeros relative to the non-zero digits determines their significance.

0.00100100 answer: 6 sig figs

Zeros to Middle, To the right, only if DECIMAL

the left, ALWAYS POINT appears anywhere

NEVER

significant

0.005 ______ 0.0050 _______ 500 _______ 500. _______ 500.0 _______

For calculations, it’s like “finding the weakest link”.

To ADD or SUBTRACT, To MULTIPLY or DIVIDE,

count decimal places. count sig figs.

5.2 one decimal place 2 Sig Figs 150 = 54.94505…

+ 3.01 3 Sig Figs 2.73

+ 6.095

14.305

Answer: 14.3 rounded off to one Answer: 55 rounded off to 2 Sig Figs

decimal place

ex) 2.028 – 0.73 = ____________ ex) 32.68 (0.100) = 45.32 (x)

The % water in a hydrate was experimentally measured to be 52%. The accepted value is 36%. What is the

percent error?

% error = measured value - accepted value x 100 %error = 52 - 36 x 100 = 44.%

accepted value 36