empirical formulas. methanal vs. acetic acid both substances have c:h:o in a 1:2:1 ratio, but they...

EMPIRICAL FORMULAS

Methanal vs. acetic acid

Both substances have C:H:O in a 1:2:1 ratio, but they are completely different substances.

Methanal vs. acetic acid

The EMPIRICAL FORMULA for both these substances is CH2O.

The MOLECULAR FORMULA for acetic acid is C2H4O2.

EMPIRICAL VS. MOLECULAR FORMULAS

EMPIRICAL FORMULAS show the simplest whole number ratio of elements in a compound.

MOLECULAR FORMULAS show the element symbols and the exact no. of each type of atom in a molecular compound.

**formulas for ionic compounds are ALWAYS empirical.

Comparing Empirical & Molecular Formulas

• Sometime the empirical and molecular formulas are the same and sometimes they are different

Glucose Hydrogen peroxide

Hydrazine Water

Molecular Formula C6H12O6 H2O2 N2H4 H2O

Empirical Formula

Finding the Empirical Formula from Percent Composition

1.Use a chart to organize your work\

2.Assume you have 100 g sample so that your mass % is the same as your mass in grams.

3.To get a whole number at the end…multiply.

Atom Assume 100g:Mass (g)

MM (g/mol) n (mol) Smallest Ratio

Whole Number

A rhyme to help you remember what goes in the chart:

Percent to massMass to moleDivide by smallMultiply til whole

If the decimal ends in…. Fraction Multiply by…...33 1/3 3…25 1/4 4... 1.5 ½ 2…,67….75

EXAMPLE 1: Find the empirical formula of a compound with 35.4% sodium and the remainder nitrogen.

STEP 1: Always assume you are working with 100 g of the compound.



Whole Number

Na 35.4g 22.99

N 64.6g 14.01


STEP 2: Calculate the amount of each element in moles using n=m/MM



Whole Number

Na 35.4g 22.99 n=m/MM =35.4/22.99 =1.539…

N 64.6g 14.01 n=4.61…


STEP 3: Determine the whole number ratio by dividing each element mole value by the lowest number.



Whole Number

Na 35.4g 22.99 n=m/MM =35.4/22.99 =1.539…

1

N 64.6g 14.01 n=4.61…3


STEP 4: Multiply each number by an integer to obtain all whole numbers.

Empirical Formula is NaN3**not needed this time


MM (g/mol)

n (mol) Smallest Ratio

Whole Number

Na 35.4g 22.99 n=m/MM =35.4/22.99 =1.539…

1

N 64.6g 14.01 n=4.61…3

EXAMPLE 2: Determine the empirical formula of a compound that contains 69.9% iron and 30.1% oxygen by mass.

***Assume 100g of substance***

STEP 1: Calculate the amount of each element in 100g.


STEP 2: Divide the amount of each element by the smallest amount.


STEP 3: Find an equivalent whole number ratio and write the empirical formula.

Tips for Solving Empirical Formula Problems• Don’t round until the very end.

• If the value is 0.95 round to 1, if the value is 0.05 round to 0, if between 0.45 to 0.55 round to 0.5.

Determining the Empirical Formula by Experiment • You can calculate a molecular formula

using the periodic table, for example you know that pure copper will oxidize to become CuO. But, as a scientist you need to confirm this by experimentation.

• This can be done using a variety of methods such as a synthesis reaction.

HOMEWORK:pp. 292 #1 (practice)

pp. 293 #1-5, 7

Molecular Formula

• The molecular formula (actual formula) can only be found if you have both the:o Molar mass of the compound &o Empirical Formula (or percent composition)

Steps to Finding Molecular Formula1.Find the molar mass of the empirical formula

2.Find the ratio of the molar mass of the compound to the empirical formula using:

3. Multiply the empirical formula by the ratio to obtain the molecular formula.

Example 1

• The empirical formula for Vitamin C is C5H4O3 and its molar mass is 176.17 g/mol. Find the molecular formula

• MMEF = 88.07 g/mol

• Ratio = MMMF = 176.17g/mol = 2

MMEF 88.07g/mol

• Molecular Formula = C5 x 2 H4 x 2 O3 x 2

= C10H8O6

Learning Check A compound with an empirical formula of C2OH4 and a molar mass of 88g/mol. What is the molecular formula of the compound?

C4O2H8

Exit Card

• A sample of indium chloride with a mass of 0.5000g is found to have 0.2404g of chlorine. What is the empirical formula of the indium compound?

empirical formulas. methanal vs. acetic acid both substances have c:h:o in a 1:2:1 ratio, but they...

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