This question is about olive oil - AQA - A-Level Chemistry - Question 9 - 2022 - Paper 2

The amount of bromine (Br₂) in the target titre can be calculated using the formula:

$ext{moles} = ext{concentration} imes ext{volume}$

Here, the concentration is 0.025 mol dm⁻³ and the volume is 30.0 cm³ (which is converted to dm³ as 0.030 dm³).

So, $ext{moles} = 0.025 imes 0.030 = 0.00075 ext{ mol}$

We know from the previous calculation that 0.00075 mol of Br₂ is decolorized. As the reaction between unsaturated fat Y and bromine occurs in a 1:1 mole ratio, we also have:

• Moles of Y = 0.00075 mol.
• The molar mass of Y (C18H34O2) is approximately 284 g/mol.

Now, calculate the mass of Y: $ext{mass of Y} = ext{moles} imes ext{molar mass} = 0.00075 imes 284 ext{ g} ext{ (for Y)} = 0.213 g$

Since the olive oil contains 85% of Y, the mass of olive oil needed is: ext{mass of olive oil} = rac{0.213 ext{ g}}{0.85} ext{ ≈ 0.250 g}

An extra step would be to ensure that the weighing bottle is clean and dry before placing the olive oil inside it. This will prevent any contamination or residual solvent from affecting the measured mass.

The flask is inverted several times to ensure that the olive oil and solvent are thoroughly mixed, allowing for a homogeneous solution.

The ion with m/z = 345 corresponds to the mass of the compound formed. To determine the molecular formula, we subtract the molar mass of hydride ion (1 g/mol) from 345, giving us 344 g/mol for the compound's neutral form. The empirical formula CxHyO can be deduced by dividing the total molecular weight by the weight of one empirical unit. Further working will show:

• By determining common ratios for C, H, and O from the fragment ion data and thus deducing that the molecular formula is likely C20H40O.