Year 12s: what will be the first way you research your future options? >>

OCR A Chemistry - Few Questions

1. Can someone guide me into how answering questions that ask you to work out the percentage composition of X in a compound Y. I can't link some questions at the moment but it's those type of questions where you do a titration and you want to work out the moles of something in the original 250cm^3.

What pattern do you guys follow to answer these questions?

2. I don't understand those questions when they remove an electron and say why there is a sharp increase in IE. For example Rb and Sr. Sr has a higher first IE than Rb but Rb has a higher 2nd IE than Sr. Why? How do you know this? I know it's something to do with the electrons being closer or something but can someone explain this.

Thank you :smile:

Reply 1

Zebash04

For your first question it is difficult to tell without an example because there are percentage by mass questions and then there are moles questions. So which one do you mean? But the general approach we were told to take was to work out the number of moles of whatever is given and then calculate a ratio. If you have an example, I could try to give a more specific answer.

For the IE question: Sr has a higher first IE than Rb because across a period the first IE always increase. This is due to an increase in nuclear charge (more protons, so a greater pull to the positively charged nucleus is experienced by the outermost electrons). Shielding remains the same and atomic radii decrease slightly across a group. Therefore, more energy is required to remove the outermost electron in Sr. For the second IE, Sr has a lower IE as the second electron is removed from the same 5s orbital, while in Rb it is removed from a new shell, closer to the nucleus (4p). This means that there is more shielding in Rb so more energy required to remove the second outermost electron.Hope this helps :smile: