This question involves understanding acid-base titrations and the correct use of indicators, specifically methyl orange. Methyl orange is a common indicator used in titrations; it changes color depending on the pH. It is red in acidic conditions (pH < 3.1) and yellow in basic conditions (pH > 4.4). The end point is the transition between these colors, typically observed as a change from pinkish-red to yellow or vice versa, depending on the direction of titration.

When an alkali (base) is titrated against an acid, we are adding acid to the base. Methyl orange is suitable for strong acid-strong base titrations and strong acid-weak base titrations because its pH range (3.1-4.4) falls within the sharp pH change region of these titrations.

Let's evaluate the options:

Option 1: Weak base vs. Strong acid - End point: Colourless to pink. Methyl orange does not give a colourless to pink change; it is red in acid and yellow in base. This is incorrect.

Option 2: Strong base vs. Strong acid - End point: Pinkish red to yellow. In this titration, adding acid to base, the solution starts basic (yellow with methyl orange) and becomes acidic at the end point, changing to pinkish-red. However, the end point described is pinkish red to yellow, which would be for adding base to acid, not acid to base. Since the question says "alkali is titrated against an acid," it means acid is added to alkali, so the color change should be from yellow (basic) to pinkish red (acidic). This option describes the reverse, so it is incorrect for this titration direction.

Option 3: Strong base vs. Strong acid - End point: Pink to colourless. Methyl orange does not have a colourless state; its colors are red and yellow. This is incorrect.

Option 4: Weak base vs. Strong acid - End point: Yellow to pinkish red. For a weak base vs. strong acid titration, methyl orange is appropriate. As acid is added to the weak base, the solution starts yellow (basic) and at the end point changes to pinkish red (acidic). This matches the direction and the indicator behavior.

Thus, the correct combination is a weak base titrated with a strong acid, with methyl orange indicator, giving an end point from yellow to pinkish red.

Related Topics and Formulae

pH Range of Indicators: The choice of indicator depends on its pH transition range relative to the pH change at the equivalence point. For methyl orange, pH range is 3.1 (red) to 4.4 (yellow).

Equivalence Point pH: For strong acid-strong base, equivalence point is at pH 7. For strong acid-weak base, equivalence point is acidic (pH < 7), which is why methyl orange is suitable.

Titration Curve: The steepness of the pH change near the equivalence point determines indicator suitability. Methyl orange works well when the end point pH fall is within 3.1-4.4.