Tap water contains a number of dissolved chemicals - Scottish Highers Chemistry - Question 7 - 2023

The term used to describe this part of the molecule is hydrophilic.

To calculate the concentration of calcium ions:

1. Determine moles of EDTA used:

   0.0045 imes ext{9.3 cm}^3 imes rac{1 ext{ mol}}{1000 ext{ cm}^3} imes ext{(1 mole of Ca}^{2+} ext{ : 1 mole of EDTA)}

2. Use the titration formula to calculate concentration:

7.415 imes 10^{-3} ext{ mol of } Ca^{2+} ext{ in 50 cm}^3\ ext{Thus, concentration = } rac{{7.415 imes 10^{-3} ext{ mol}}}{{0.050 ext{ dm}^3}} = 0.000837 ext{ mol}^-1.

Therefore, the concentration of calcium ions in the tap water is approximately 0.000837 mol⁻¹.

To estimate the concentration of permanganate ions from the graph:

1. Locate the absorbance value on the y-axis:

   • Find the value of 0.08 on the absorbance axis.

2. Draw a horizontal line to intersect the curve:

   • Move horizontally from 0.08 until you intersect the curve.

3. Read the corresponding concentration on the x-axis:

   • From the graph, the concentration at this point is approximately 4.4 to 5.4 mg dm⁻³.

The difference in polarities arises due to the molecular structure and electronegativity of the chlorine and hydrogen atoms:

1. Trichloromethane (CHCl₃):

   • It has a polar C-H bond and a permanent dipole due to the electronegative chlorines. The molecule is overall polar.

2. Tetrachloromethane (CCl₄):

   • The four chlorine atoms are symmetrically arranged around the carbon atom, leading to a non-polar molecule despite the C-Cl bonds being polar. The dipoles cancel out, resulting in no net dipole moment.

To calculate the enthalpy change:

1. Identify bond enthalpies from the data booklet for:

   • C-H, C-Cl, and H-F.

2. Use the formula:

   • ext{(Total bonds broken)} + ext{(Total bonds formed)}

3. Calculate resulting enthalpy change:

   • For the reaction of trichloromethane with H-F, after plugging in the values, you should arrive at an enthalpy change of approximately -14 kJ mol⁻¹.