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Percent Composition: Flashcards

Revise formula mass, mass percent and the most common mistakes.

9 cardsBeginner

9 cards

  1. Question
    What does percent composition tell you?
    Answer

    The percentage of a compound's mass that comes from each element.

  2. Question
    How do you calculate the mass percent of an element in a compound?
    Answer

    (number of atoms of the element × atomic mass) ÷ formula mass × 100%

  3. Question
    What is a formula mass?
    Answer

    The sum of the atomic masses of all the atoms in a chemical formula, in u.

  4. Question
    What is the formula mass of Ca(OH)X2\ce{Ca(OH)2}? (Ca = 40.08, O = 16.00, H = 1.008)
    Answer

    40.08+2(16.00+1.008)=74.1040.08 + 2(16.00 + 1.008) = 74.10 u. The 2 multiplies both O and H.

  5. Question
    What is the mass percent of hydrogen in water, HX2O\ce{H2O}?
    Answer

    2(1.008)18.016×100%=11.19%\dfrac{2(1.008)}{18.016} \times 100\% = 11.19\%

  6. Question
    Two-thirds of the atoms in water are hydrogen. Why is hydrogen only 11.19% of its mass?
    Answer

    Hydrogen atoms are much lighter than oxygen atoms. Percent composition is by mass, not by number of atoms.

  7. Question
    Does the percent composition of a pure compound depend on the size of the sample?
    Answer

    No. A pure compound always has the same composition by mass, so a drop and a lake of pure water have the same percentages.

  8. Question
    What should the mass percents of all elements in a compound add up to?
    Answer

    100%, apart from small rounding differences. Keep extra digits until the end to avoid them.

  9. Question
    What is relative formula mass, MrM_\text{r}?
    Answer

    The formula mass expressed without units, relative to 112\frac{1}{12} of the mass of a carbon-12 atom. It has the same number as the formula mass in u.