Question

6
phenyl magnesium bromide is used as a grignard reagent in organic synthesis. determine its molecular formula if its molar mass is 181.313 g/mol and it contains 39.7458 % c, 2.77956 % h, 13.4050 % mg, and 44.0697 % br. *
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7
three naturally occurring iron compounds are pyrite (fes2), hematite (fe2o3) and siderite (feco3). which contains the greatest percentage of iron? *
○ pyrite
○ hematite
○ siderite

Explanation:

Question 6

Step1: Calculate moles of each element

Assume we have 100g of the compound. Then:

• Mass of C: \( 39.7458\space g \), moles of C: \( \frac{39.7458}{12.01} \approx 3.309\space mol \)
• Mass of H: \( 2.77956\space g \), moles of H: \( \frac{2.77956}{1.008} \approx 2.758\space mol \)
• Mass of Mg: \( 13.4050\space g \), moles of Mg: \( \frac{13.4050}{24.305} \approx 0.5515\space mol \)
• Mass of Br: \( 44.0697\space g \), moles of Br: \( \frac{44.0697}{79.904} \approx 0.5515\space mol \)

Step2: Find the mole ratio

Divide each by the smallest number of moles (0.5515):

• C: \( \frac{3.309}{0.5515} \approx 6 \)
• H: \( \frac{2.758}{0.5515} \approx 5 \) (approx, let's check with molar mass)

Wait, molar mass is 181.313 g/mol. Let's use molar mass to find actual moles.

Moles of C: \( 0.397458 \times 181.313 / 12.01 \approx 6 \)
Moles of H: \( 0.0277956 \times 181.313 / 1.008 \approx 5 \)
Moles of Mg: \( 0.134050 \times 181.313 / 24.305 \approx 1 \)
Moles of Br: \( 0.440697 \times 181.313 / 79.904 \approx 1 \)

So the formula is \( C_6H_5MgBr \)

To find the percentage of iron in each compound, use the formula: \( \% Fe = \frac{\text{Molar mass of Fe in compound}}{\text{Molar mass of compound}} \times 100 \)

• Pyrite (FeS₂): Molar mass of FeS₂ = \( 55.85 + 2 \times 32.07 = 119.99\space g/mol \). \( \% Fe = \frac{55.85}{119.99} \times 100 \approx 46.55\% \)
• Hematite (Fe₂O₃): Molar mass of Fe₂O₃ = \( 2 \times 55.85 + 3 \times 16.00 = 159.70\space g/mol \). \( \% Fe = \frac{2 \times 55.85}{159.70} \times 100 \approx 69.94\% \)
• Siderite (FeCO₃): Molar mass of FeCO₃ = \( 55.85 + 12.01 + 3 \times 16.00 = 115.86\space g/mol \). \( \% Fe = \frac{55.85}{115.86} \times 100 \approx 48.20\% \)

Comparing the percentages, hematite has the highest percentage of iron.

Answer:

\( C_6H_5MgBr \)

Question 7