A41-g sample of potassium completely reacts with chlorine to form 78 g of potassium chloride. how many grams of chlorine must have reacted?

37.275g must react

Explanation:

Step 1: Balance the reaction

The reaction is as follow:

2 K + Cl2> 2KCl

This means that for 2 moles K we have 1 mole Cl2 ( and 2 moles KCl)

Step 2: Calculate moles

Moles of potassium:  moles = mass / Molar mass

Moles of potassium = 41g / 39.1g/mol  

Moles of potassium = 1.05 moles

for each 2 moles potassium we have 1 mole Cl2 ( and 2 moles KCl)

Moles of Chlorine : 1.05 / 2 = 0.525 moles ( and 1.05 moles of KCl)

Step 3: Calculate mass of chlorine

mass chlorine = moles x Molar mass  = 0.525 x 35.5 x 2

       = 37.275g Chlorine

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.