SURVEY OF CHEMISTRY LABORATORY I

CHEM 1151L

EMPIRICAL FORMULA OF ZINC CHLORIDE

CAUTION!!!CAUTION!!!BE SURE TO WEAR SAFETY GLASSES AT

ALLTIMES IN THE LABORATORY

NO EXCEPTIONS TO THIS RULE!

Students will determine

- the empirical formula of zinc chloride by

a process known as synthesis

- the empirical formula from their data

- the empirical formula from the class average

It is best to perform several experiments and obtain average values for the result

EMPIRICAL FORMULA OF ZINC CHLORIDE

- The lowest whole number ratio of atoms present in a compound

- the lowest whole number ratio of moles of atoms in a compound

For example

- The molecular formula of glucose is C6H12O6

- The empirical formula is CH2O

EMPIRICAL FORMULA

Can be determined by the following methods

Analysis

- A compound is broken down into its element

- The mount of each element present is accurately determined

Synthesis

- Prepared from accurately measured amounts of each of its

elements

EMPIRICAL FORMULA

- Determine the mass of each element found in the formula

(may be grams or percentage composition by mass)

- Convert each mass to moles using the atomic masses of the appropriate elements

- Divide each mole amount by the smallest number of moles

- Round each mole ratio to the nearest integer

- Write the whole numbers as subscripts in the formula

EMPIRICAL FORMULA

EMPIRICAL FORMULA

mol231.6g/mol12.01

g74.83 mol97.24

g/mol1.01

g25.17

000.1mol 6.231

mol 6.231 007.4

mol 6.231

mol 24.97

Assume 100.0 g sample

% of each element equals grams

C H

1.000 ≈ 1 4.007 ≈ 4

Empirical Formula: CH4

Determine the empirical formula of a compound whose composition is determined to be 74.83% carbon and 25.17% hydrogen

ZINC METAL

- Mass about 1 gram of zinc metal

- Record this mass

EVAPORATING DISH

- Mass a clean, dry evaporating dish

- Record this mass

HYCHLORIC ACID

- Use the automatic dispensing bottle

- Dispense 10 mL of 12 M hydrochloric acid (HCl) directly

into the evaporating dish

HYCHLORIC ACID + ZINC METAL

- Carefully add the zinc to the dish containing the HCl

- Must be done in the hood

- Allow all of the zinc to dissolve

EVAPORATE LIQUID

- Gently warm solution on a hotplate

- Must be done in the hood

- Avoid spattering

- A white powdery substance (zinc chloride) will form

COOL AND MASS PRODUCT

- Avoid melting of zinc chloride

- Take the dish off the hotplate when content is dried enough

- Allow the dish to cool

COOL AND MASS PRODUCT

- Mass the dish and the zinc chloride

- Record the mass

CALCULATIONS

- Complete your calculations on the data sheet

- Report the percent by mass of each element to the instructor before leaving the lab

- The class average is needed to complete the lab report