Vitamin C has the formula C_{6}H_{8}O_{6}. Calculate the number of vitamin C molecules present in a 0.250-g tablet of pure vitamin C.

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Step 1: The given quantity is 0.250 g of C_{6}H_{8}O_{6}, and the desired quantity is molecules of C_{6}H_{8}O_{6}.

\quad\quad\quad\quad0.250 g C_{6}H_{8}O_{6} = ? molecules C_{6}H_{8}O_{6}

In terms of Figure 6.7, this is a “grams of A” to “particles of A” problem. We are given grams of a substance, A, and desire to find molecules (particles) of that same substance.

Step 2: Figure 6.7 gives us the pathway we need to solve this problem. Starting with “grams of A,” we convert to “moles of A” and finally reach “particles of A.” The arrows between the boxes along our path give the type of conversion factor needed for each step.

\quad\quad\quad\quad \boxed{Grams of A} \xrightarrow[mass]{Molar} \boxed{Moles of A} \xrightarrow[number]{Avogadro’s} \boxed{Particles of A}

Using dimensional analysis, the setup for this sequence of conversion factors is

\quad\quad\quad\quad 0.250 \cancel{g C_{6}H_{8}O_{6}}\times (\frac{1 moles C_{6}H_{8}O_{6}}{176.14 \cancel{g C_{6}H_{8}O_{6}}})\times (\frac{6.02\times 10^{23} molecules C_{6}H_{8}O_{6}}{1 \cancel{moles C_{6}H_{8}O_{6}}})

\quad\quad\quad\quad\quad\quad\quadg C_{6}H_{8}O_{6} → moles C_{6}H_{8}O_{6} → molecules C_{6}H_{8}O_{6}

The number 176.14 that is used in the first conversion factor is the formula mass of C_{6}H_{8}O_{6}. It was not given in the problem but had to be calculated by using atomic masses and the method for calculating formula masses shown in Example 6.1.

Step 3: The solution to the problem, obtained by doing the arithmetic, is

\quad\quad\quad\quad\quad \frac{0.250\times 1\times 6.02\times 10^{23}}{176.14\times 1} molecules C_{6}H_{8}O_{6}\\\quad\quad\quad\quad\quad\quad\quad = 8.54\times 10^{20} molecules C_{6}H_{8}O_{6}