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Chapter 4: Problem 156

Tris(pentafluorophenyl)borane, commonly known by its acronym BARF, is frequently used to initiate polymerization of ethylene or propylene in the presence of a catalytic transition metal compound. It is composed solely of C, F, and B; it is 42.23% C and 55.66% F by mass. a. What is the empirical formula of BARF? b. A 2.251-g sample of BARF dissolved in 347.0 mL of solution produces a 0.01267-M solution. What is the molecular formula of BARF?

Short Answer

Expert verified
The empirical formula of BARF is C18BF15. The molecular formula of BARF is also C18BF15.

Step by step solution

01

Convert mass percent to grams

Assume we have a 100-g sample of BARF. The sample contains 42.23 g of C and 55.66 g of F. We can then calculate the mass of B. Mass of B = 100 g - (42.23 g of C + 55.66 g of F) = 100 g - 97.89 g = 2.11 g
02

Convert grams to moles

Now, we will convert the grams of each element to moles. Moles of C = \( \frac{42.23 g}{12.01 g/mol} \) ≈ 3.517 mol Moles of F = \( \frac{55.66 g}{19.00 g/mol} \) ≈ 2.930 mol Moles of B = \( \frac{2.11 g}{10.81 g/mol} \) ≈ 0.195 mol
03

Determine the mole ratio

To find the empirical formula, we need to determine the mole ratio of each element. Mole ratio of C to B to F = \( \frac{3.517 mol}{0.195 mol} : \frac{0.195 mol}{0.195 mol} : \frac{2.930 mol}{0.195 mol} \) ≈ 18 : 1 : 15
04

Write the empirical formula

Based on the mole ratio, the empirical formula of BARF is C18BF15. #b. Determine the molecular formula of BARF#
05

Calculate the molecular weight from the given data

We are given the volume of the solution (347.0 mL) and its molarity (0.01267 M). Using these values, we can calculate the moles of BARF in the 2.251-g sample: Moles of BARF = Molarity × Volume = 0.01267 M × 0.347 L = 0.00439 mol To find the molecular weight, we can divide the mass of the sample by the moles: Molecular weight of BARF = \( \frac{2.251 g}{0.00439 mol} \) ≈ 513 g/mol
06

Compare molecular weight and empirical formula weight

Next, we need to calculate the empirical formula weight: Empirical formula weight of C18BF15 = (18 × 12.01) + (1 × 10.81) + (15 × 19.00) ≈ 513 g/mol
07

Determine the molecular formula

Since the molecular weight of BARF (513 g/mol) and the empirical formula weight of C18BF15 (513 g/mol) are identical, the empirical formula and the molecular formula of BARF are the same. The molecular formula of BARF is C18BF15.

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Most popular questions from this chapter

A \(2.20-\) g sample of an unknown acid (empirical formula \(=\) $\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{O}_{3} )\( is dissolved in 1.0 \)\mathrm{L}$ of water. A titration required 25.0 mL of 0.500 M NaOH to react completely with all the acid present. Assuming the unknown acid has one acidic proton per molecule, what is the molecular formula of the unknown acid?

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