experiment 11

A. Finding the Simplest Formula


  • The simplest formula of a compound represents the lowest whole-number ratio of the atoms in that compound.


  • Example of Compounds and Their Formulas:

    • Benzene:


    • Molecular formula: C_6H_6


    • Simplest formula: CH


    • Others:

      Compound Name

      Molecular Formula

      Simplest Formula


      Acetylene

      C_2H_2

      CH


      Benzene

      C_6H_6

      CH


      Ammonia

      NH_3

      NH_3


      Hydrazine

      N_2H_4

      NH_2

      • To determine the simplest formula:

      • Convert the grams of each element to moles.

      • Find the lowest whole-number ratio to use as subscripts in the formula.

      • Example Calculation:

      • Given:

      • 0.040 moles of Zn

      • 0.080 moles of Cl

      • Step 1: Divide each element's moles by the smaller number of moles:

      • rac{0.080 ext{ mole Cl}}{0.040} = 2 ext{ moles of Cl}

      • rac{0.040 ext{ mole Zn}}{0.040} = 1 ext{ mole of Zn}

      • Step 2: Use the whole numbers as subscripts to write the formula:

      • ZnCl_2 (the subscript 1 for Zn is understood).

      B. Formula of a Hydrate

      • Definition: A hydrate is an ionic compound that is combined with a fixed specific number of water molecules.

      • This number of water molecules may differ from one hydrate to another.

      • The format to express the number of water molecules is as follows:

        • Written after the ionic formula and separated by a raised dot (•).

        • Examples of Hydrate Formulas:

        • Calcium Sulfate Dihydrate: CaSO_4 ullet 2H_2O

        • Copper(II) Sulfate Pentahydrate: CuSO_4 ullet 5H_2O

        • Sodium Carbonate Decahydrate: Na_2CO_3 ullet 10H_2O

      • Removal of Water:

        • Water molecules in a hydrate can be removed by heating.

        • After heating, the remaining ionic compound is termed an anhydrate.

        • Example reaction:

        • When heating Copper(II) Sulfate Pentahydrate: CuSO_4 ullet 5H_2O ightarrow CuSO_4 + 5H_2O(g)

          • The removed water is referred to as water of hydration.

      • Experimentally Determining the Amount of Water:

        • Measure the mass of the hydrate before heating and the mass of the anhydrate after heating.

        • The difference in mass reflects the amount of water removed, which is the water of hydration.

      • Example Calculation of Percent Water in a Hydrate:

        • Given a sample of 2.00 ext{ g} of CuSO_4 ullet 5H_2O heated to produce 1.28 ext{ g} of anhydrate CuSO_4:

        1. Calculate grams of water lost:

        • 2.00 ext{ g of hydrate} - 1.28 ext{ g of anhydrate} = 0.72 ext{ g of } H_2O ext{ in hydrate}

        1. Calculate percent of water in the hydrate:

        • rac{0.72 ext{ g } H_2O}{2.00 ext{ g } CuSO_4 ullet 5H_2O} imes 100 ext{%} = 36 ext{% } H_2O ext{ in hydrate}