chemical math - chemistry revie · chemical math formula writing ... criss-cross and use...
TRANSCRIPT
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
-----------------------------------------------------------------------------------------------------------------
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