A-Level Edexcel Chemistry Already Passed

Why might percentage yield value not be 100% {Ans: Incomplete reactions, side reactions, losses during transfer of substances, losses during purification}What is hydrogen stored in fuel cells as? {Ans: Liquid water under pressure Adsorbed on surface of solid Adsorbed within a solid}Procedure for making a standard solution {Ans: 1. weigh sample bottle on mass balance 2. re-weigh sample bottle after adding required mass of solid on 2D.P mass balance and then transfer to breaker 2. Record difference in mass 3. Add 100cm^3 of distilled water to the beaker and use glass rod to dissolve solid (if solid does not dissolve well in cold water, heat gently) 4. Pour solution into 250cm^3 graduated/volumetric flask via funnel 5. Rinse beaker and funnel and add washings from breaker and glass rod to the flask 7. Make up to the mark with distilled water using a pipette for the last few drops 8. Invert flask several times to ensure uniform solution Fill so bottom of meniscus sits on line (may be difficult with darker liquids)}Advantages of hydrogen fuel cells over conventional petrol or diesel powered vehicles? {Ans: Less pollution and CO2 (only emits water) greater efficiency Hydrogen gas readily available