Kcse 2006 Chemistry Paper 3 Marking Scheme -

The relative formula mass of the unknown solid based on a specific mole ratio (e.g., 2 moles of manganate reacting with 5 moles of the substance). Penalties for Formatting: In the qualitative analysis sections, half-marks ( ) are often penalized for omitting or providing incorrect state symbols in chemical equations. Typical Scoring Breakdown Common Marks Table Completion Accuracy within of the "school value" 1 - 3 Marks Graph Work Proper labeling of axes and a smooth curve Calculations Correct use of formulas (e.g., or molarity formulas) 1 Mark each Observations

| Step | Action | Marks | | :--- | :--- | :--- | | 1 | Moles of Na₂CO₃ in 25 cm³ pipette = (Molarity x Volume)/1000 | 2 | | 2 | Mole ratio (Na₂CO₃ : HCl) – Usually 1:2 | 1 | | 3 | Moles of HCl used = 2 x (moles of Na₂CO₃) | 2 | | 4 | Concentration of HCl = (Moles of HCl x 1000) / Average Titre | 3 | | 5 | Correct significant figures (4 d.p for moles, 2 d.p for conc.) | 2 | | | | 10 | kcse 2006 chemistry paper 3 marking scheme

A graph of solubility (g/100g of water) against temperature was required. The relative formula mass of the unknown solid