Questions: Chemical Reactions Finding mole ratios from chemical formulae Brookelyn This is the chemical formula for talc (the main ingredient in talcum powder): Mg3(Si2O5)2(OH)2 An analytical chemist has determined by measurements that there are 35 moles of silicon in a sample of talc. How many moles of hydrogen are in the sample? Be sure your answer has the correct number of significant digits. mol × 10

Solution Steps

The chemical formula for talc is: \[ \mathrm{Mg}_{3}\left(\mathrm{Si}_{2} \mathrm{O}_{5}\right)_{2}(\mathrm{OH})_{2} \]

This indicates that each formula unit of talc contains:

• 3 moles of Mg
• 4 moles of Si (since \(\left(\mathrm{Si}_{2} \mathrm{O}_{5}\right)_{2}\) means 2 units of \(\mathrm{Si}_{2} \mathrm{O}_{5}\), each containing 2 Si atoms)
• 10 moles of O (5 from each \(\mathrm{Si}_{2} \mathrm{O}_{5}\) unit, and there are 2 such units)
• 2 moles of H (from \((\mathrm{OH})_{2}\))

From the chemical formula, we see that for every 4 moles of Si, there are 2 moles of H. Therefore, the mole ratio of Si to H is: \[ \frac{4 \text{ moles Si}}{2 \text{ moles H}} = 2 \text{ moles Si per mole H} \]

Given that there are 35 moles of silicon in the sample, we use the mole ratio to find the moles of hydrogen: \[ \text{Moles of H} = \frac{\text{Moles of Si}}{2} = \frac{35 \text{ moles Si}}{2} = 17.5 \text{ moles H} \]

Final Answer