A 1.50 g sample of a compound containing only C, H and O was burned in an attempt to determine its simplest formula. 3.00 g of CO2 and 1.23 g of H2O were collected. Find the simplest formula for the compound.

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Explanation:

The mass of carbon and hydrogen is calculated from the mass of their oxides ([tex]CO_{2}[/tex] and [tex]H_{2}O[/tex]) as follows.

    Mass of C = [tex]3.00 g CO_{2} \times \frac{12.01 g C}{44.0 g CO_{2}}[/tex]

                      = 0.818 g C

    Mass of H = [tex]1.23 g CO_{2} \times \frac{2.016 g H}{18.02 g H_{2}O}[/tex]

                      = 0.137 g H

So, the mass of C + mass of H is as follows.

                        0.818 g + 0.137 g = 0.955 g

This mass is actually less than the mass of sample. And, the missing mass must be caused by O. Hence, the mass of O will be calculated as follows.

            Mass of O = 1.50 g - 0.955 g

                              = 0.545 g

Now, we convert masses to moles and find their moles and ratio as follows.

  Element     Mass/g      Moles    Ratio    [tex]\times 2[/tex]     Integers

       C             0.818       0.068       1        2              2

       H             0.137        0.068      1        2               2

       O             0.545       0.0340    0.5   1                1

Thus, we can conclude that simplest formula for the given compound is [tex]C_{2}H_{2}O[/tex].

The simplest formula of the compound is CH2O.

We must obtain the number of moles of carbon, hydrogen and oxygen as follows;

Mass of carbon = 3.00 g × 12g/44 g = 0.82 g

Number of moles of carbon =  0.82 g/12 g/mol = 0.068 moles

Mass of hydrogen = 1.23 g × 2/18 = 0.14 g

Number of moles of hydrogen = 0.14 g/ 1 g/mol = 0.14  moles

Mass of oxygen = 1.5g - (0.82 g +  0.14 g) = 1.39 g

Number of moles of oxygen = 1.39g/16g/mol = 0.087 moles

Now, we have to divide through by the lowest ratio;

C - 0.068/0.068  H -  0.14/0.068     O = 0.087/0.068

C - 1   H - 2  O- 1

The simplest formula of the compound is CH2O.

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