What is the mass of potassium chloride when 6.75g of potassium reactants with an excess of chlorine gas the balanced chemical equation

2K+Cl₂→2KCl
First you need to find the number of moles of potassium that is present in a 6.75g sample.  To do this you need to divide the mass of potassium (6.74g) by its molar mass (39.1g/mol) to get 0.172mol K.  Since we are told that the reaction happens with excess chlorine we know that potassium is the limiting reagent and we can therefore ignore chlorine gas and convert the moles of potassium straight to the moles of potassium chloride.  Since we know that 2 moles of potassium creates 2 moles of potassium chloride we can say that 0.172mol K turns into 0.172mol KCl since the molar ratio is 2/2 which can be thought of as 1.  The final step is to multiply 0.172mol KCl by its molar mass (74.55g/mol) to get 12.85g of potassium chloride.

I hope this helps.  Let me know in the comments if anything is unclear.