Writing and balancing equationsIB MYP Chemistry: Mind map

Exam questions on Writing and balancing equations

1. In a school laboratory in Dubai, students add small pieces of zinc to dilute hydrochloric acid in a test tube. Bubbles of hydrogen gas form and the zinc slowly disappears, leaving a colourless solution of zinc chloride.
   Explain why the number 2 is written in front of HCl in the balanced equation.2 marks
2. A student in Lagos places a conical flask containing marble chips (calcium carbonate) and dilute hydrochloric acid on a balance. Cotton wool is pushed into the neck of the flask. As the reaction takes place the balance reading falls by 0.64 g, even though no solid or liquid has left the flask.
   A different student does the same reaction in a strong sealed container. Explain why the total mass would not change.2 marks
3. A student investigates whether mass changes during a reaction. She places a small test tube of sodium carbonate solution inside a conical flask containing calcium chloride solution, seals the flask with a stopper and weighs it. She tips the flask so that the two solutions mix and a white precipitate of calcium carbonate forms. She then weighs the sealed flask again. Her balance reads to 0.01 g. The mass of the sealed flask before and after mixing was: trial 1, 215.32 g then 215.32 g; trial 2, 187.45 g then 187.45 g; trial 3, 201.18 g then 201.17 g.
   State a testable hypothesis for this investigation, give a scientific reason for it, and identify the dependent variable.3 marks