Chemistry 11 – Mole Concept Study Guide 12

Example: The sample you synthesized gave the following analysis:

A 50 g sample contains 24.8 g C, 2.6 g H, 14.4 g N, and 8.25 g O.

Does this analysis agree with the chemical formula of caffeine, which is C8H10N4O2 (MM= 194g/mol)? Look at the % composition of carbon in both samples! Your sample: Caffeine: The percentages indicate that your sample is _____________________!

&8×12.0=96

40C = 25%81×1000/0 = 49-6 To

40C =

9¥x 100010 = 49.4845 -

- - to

indeed caffeine

Jan Nth ( Thu )

1. Review to composition

I

Eira:

docomposition of water in

copper LII ) sulfatepehydrate C use 457Chat Soga

-

CU 504 . 5h20 to H2O = MM (

Eater)

× 10040MM L CUS 04 . 5h20 )MM = 249.69cm "

=

524%8.6-0=90×1000/0

= - 36.06902

. SG

EmpiricalFormula t Molecular Formula

HFI Organic Chemistry Fts # 3 Cp . 81 in HIB )Next class = Quiz L HI # 5)

Chemistry 11 – Mole Concept Study Guide 13

Empirical and Molecular Formula

When we are analyzing an unknown compound in a lab, we can determine the percentage composition of the compound from the data results. Using the percentage composition, we can figure out the simplest ratio of atoms in this compound, also known as the empirical formula.

• Empirical Formula: _______________________________________________________ _______________________________________________________

For example, what do CH2, C2H4, C3H6, C4H8, and C5H10 have in common? They all contain ______ as many hydrogens as carbons. The empirical formula for all of these compounds is _______. Let’s take a look at how we can determine the empirical formula from the percentage composition data. What is the empirical formula of a compound consisting of 80.0 % C and 20.0 % H?

1. Assume you have _____ of the compound. Find the mass of each element in the sample.

2. Find the number of moles of each element by dividing the mass of each element by its

molar mass.

3. Find the ratio of atoms of each element by dividing the number of moles of each element by the smallest number of moles.

Use the following visual organizer

C H % 80.0 20.0 g 80.0 20.0

mol 6.67 20.0 /small 1 2.99 ~ 3

Empirical formula CH3

Therefore, the simplest ratio of atoms is ______________. The empirical formula is ________. There are times when the ratio may not be easily rounded to a whole number. If this is the case, you need to multiply this fraction or decimal number by a number (start with x2, x3, x4…. until decimals become __.99, __.98, or __.97 ) so that it becomes a whole number again.

smallest whole - number ratio of atoms which representsthe molecular composition of a species

d

twiceCHZ THz

CHZ

100gcgram )

80.0 20.0

80.082%12.020.082 -11.0

Ca.

Hao 6.666 . --

. . MOI2 :- 6. I20.0mHz ÷ 6. I

1 3

CzHg = CHS

IC :3 H

CHS

✓Whole #

④- . 01

,- . 02

,Or - . 03

Chemistry 11 – Mole Concept Study Guide 14

Example: What is the empirical formula of a compound containing 39.0 % Si and 61.0 % O?

Si O % 39.0 61.0 g 39.0 61.0

mol 1.3879 3.8125 /small 1 2.746~2.75

Multiplier 4 10.987 ~ 11 Empirical formula Si4O11

Finding the Molecular Formula The empirical formula only gives us the simplest ratio of the atoms for each element in the compound. Thus, CH2, C2H4, C3H6, C4H8, and C5H10 all have the same empirical formula CH2. Say we were given one of these 5 organic compounds. How can we figure out which one we have for sure? We need to find the molecular formula for this compound!

• Molecular Formula: ______________________________________________________ ______________________________________________________

To find the multiple that converts the empirical formula to the molecular formula, you need to be given the molar mass of the compound. Multiple = N = Molar mass = Molar mass of compound 111221 Empirical mass Molar mass of empirical formula Once we have “N”, multiply it to the empirical formula to obtain the molecular formula!

Molecular Formula = N x (Empirical Formula Example:

A molecule has an empirical formula of HO and a molar mass of 34.0 g. What is the molecular formula? Use the following visual organizer

Empirical Molecular mass (g/mol) 17.0 34.0 formula HO ?

H2O2

39.0 61.0

39.0 2--28. I 61.02 ÷ 16.0

1. 3879003 - - -

2=1.387. . 3.8125

2÷ 1.387 - -

.

1

3×42.746955 . - - -

..

2×44 <

10.981782051 - - - . I 11

Sia On

→ could be 9-

Some whole # multiple of empirical formulathat gives the exact # of atoms of each

element in a cmpnd

To usually givenMM ( molecular formula ) = N NIE

,NEIN

MM ( empirical formula )↳ you calculate MM

N x empirical formula

↳ Maecmolecular )

X2

4817→ 34.0 Cgiven )

HO c given ) I ? 2HOX Hzoz

N= 3¥ = 2

Dolocomp

D emp formula{is more

.'

46.294 #= mole12.09C

Find the emp .formula of the cmpnd w/

Ho % To

46.2g C÷ , a 7.69g H 46.2g O

d t "

smallestd "6

MOI 3.85 7.69 2.8875

C 3.85/476902.88175

. ..

" "-

a .ae . . . . .

" " '

IIsm-. I

3Ef×3

X3 4 7.9896 - - - I 8 3

C4H8O3

HFI Finish all Q in O - Chem HS # 3

Quiz I HI # 4) next Class