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12 May, 15:42

Elemental analysis of a sugar molecule called sucrose gives the following mass percent composition: C 40.00%, H 6.72 %, O 53.28 %. The molar mass of sucrose is 342.3g/mol. Find the molecular formula of sucrose. (6 pts)

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  1. 12 May, 18:50
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    C12H22O11

    Explanation:

    1. First you have to chage percentages into grams:

    Total molar mass is equal to 100%

    100% = 342.3 g

    C:

    40.00% x (342.3 g/100%) = 136.92 g

    H:

    6.72%x (342.3 g / 100%) = 23 g

    O:

    53.28% x (342.3 g/100%) = 182.38 g

    2. Then change grams into mol:

    C:

    1 mol = 12.011 g/m

    136.92 g / (12.01ol1 g/mol) = 11.3996 mol

    H:

    1 mol = 1,008 g/mol

    23 g / (1,008 g/mol) = 22.8175 mol

    O:

    1 mol = 15.999g/mol

    182.38 g / (15.999 g/mol) = 11.3995 mol

    3. Then divide the values obtained by the smallest value: 11.3995

    C:

    11.3996 mol / 11.3995 mol = 1

    H:

    23mol / 11. 3995 mol = 2

    O:

    11.3995 mol/11.3995 mol = 1

    4. Obtein the empirical formula: CH2O

    5. Find the epirical formula molar mass:

    C = 12.011 g

    H = 2 g

    O = 16g

    CH2O = 12.011 + 2.016+15.999 = 30.026 g

    6. Then, divide the molecular formula molar mass by the empirical formula molar mass:

    molecular formula molar mass / empirical formula molar mass

    342.3 g / 30 g = 11.40

    Aproximate the number obteined to a whole number : 11

    7. Multiply the subscripst by the whole number 11:

    C (1x11) H (2x11) O (1 x 11) = C11 H22 O11

    But, the real molecular formula is C12H22O11 because of the carbon who unites the two sugars.
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