What is the percentage error? if the actual density is 8.49 and the measured density is 8.45

Can someone me with questions 15 to 17 plz.

Me i dont know what to do! the table compares the number of electrons in two unknown neutral atoms. comparison of electrons atom number of electrons a 10 d 11 use this information to determine the number of valence electrons in the atoms. which of the following correctly compares the stability of the two atoms? both are unreactive. both are highly reactive. a is unreactive and d is reactive. a is reactive and d is unreactive.

What volume would 2.25 moles of ne has occupy at stp?