Consider equation 3A+B=C+D. You react 4 moles of A with 2 moles of B. Which of the following is true?

1) Limiting reactant (L.R.) is one with higher molar mass
2) A is L.R. because you need 6 moles of A and have only 4 moles
3) B is L.R. because you have fewer moles of B than A
4) B is L.R. because 3A molecules react with each B molecule
5) Neither reactant is limiting

see below.

To determine the limiting reactant, we need to compare the moles of the reactants with the stoichiometric coefficients in the balanced equation.

Given that you have reacted 4 moles of A and 2 moles of B, we can use the stoichiometric coefficients to see what the equation predicts.

From the balanced equation 3A + B = C + D, we can see that the stoichiometric coefficient for A is 3, and the stoichiometric coefficient for B is 1.

1) Comparing the molar masses of the reactants is not a valid approach for determining the limiting reactant since it doesn't consider the stoichiometry. So, option 1 is incorrect.

2) According to the stoichiometry, for every 3 moles of A, we need 1 mole of B. So, if you have only 4 moles of A, you would need 4/3 moles of B to react with it. Since you have 2 moles of B, it is not enough to fully react with the 4 moles of A. Therefore, option 2 is correct, and A is the limiting reactant.

3) You have fewer moles of B than A, but the stoichiometry tells us that for every 3 moles of A, we only need 1 mole of B. So, option 3 is incorrect.

4) The stoichiometry of the equation tells us that 3 molecules of A react with each molecule of B. However, since we are given the amounts in moles, not molecules, this information is not applicable. So, option 4 is incorrect.

5) Since option 2 is correct, A is the limiting reactant. Therefore, neither reactant is in excess, and option 5 is incorrect.

In conclusion, the correct answer is option 2) A is the limiting reactant because you need 6 moles of A and have only 4 moles.