Idon't understand this

2a+3b—> c
if 4.0 mol of a is reacted with 4.0 mol of b which of the reactant is limiting

a) b is limiting because 4. 0 mol and 6. 0 mol are needed.
b) a is limiting because 2 is smaller than 3 (the numbers refer to the coefficients the balanced equation).
c) b is limiting because 3 is larger than 2 (the numbers refer to the coefficients in the balanced equation)
d) a is limiting because 2 moland 4. 0 mol are needed. e. neither is limiting because equal amounts (4. 0 mol) of each reactant are reacted.

TLDR: A.

Understanding the idea of chemical equations is sometimes a hard concept to grasp in general chemistry because it’s typically not explained well. What a chemical equation represents is what literally happens on the subatomic level. The coefficients in front of the reactants and products mean, number for number, what you would get if you performed the reaction. It’s like an atomic recipe - you can only “make” things when you have enough ingredients. When an ingredient runs out, you can’t produce any products anymore. The ingredient that runs out the quickest is known as the “limiting reagent”, as it literally limits what you can produce.

In the equation above, 2 equivalents of “a” and 3 equivalents of “b” are consumed to produce the products. This means that “b” gets consumed faster than “a”. To make one sample of products, you need two molecules of “a” and three molecules of “b”. If you start out with four moles of each reactant (the same number of atoms), you’ll run out of “b” before “a” because the reaction consumes more molecules of “b” than molecules of “a”. Therefore, the answer would be A. You would need 6 moles of “b” to finish the reaction and use up the remaining “a” (a 2:3 ratio, just like in the balanced equation).

Hope this helps!

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.