<u>Answer:</u> The mass of methylene blue that must be weighed is 
<u>Explanation:</u>
To calculate the mass of solute, we use the equation used to calculate the molarity of solution:

We are given:
Molarity of solution = 
Molar mass of methylene blue = 319.85 g/mol
Volume of solution = 100.0 mL
Putting values in above equation, we get:

Hence, the mass of methylene blue that must be weighed is 
Answer:
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Answer: 10 grams of Hydrogen gas will be produced theoretically.
Explanation: The equation for the reaction between zinc metal and hydrochloric acid is:

As HCl is present in excess, therefore limiting reagent is Zinc metal as it limits the formation of product.
By stoichiometry,
1 mole of Zinc metal produces 1 mole of Hydrogen gas, therefore

Amount of Hydrogen gas produced can be calculated by:

Molar mass of Hydrogen = 2 g/mol
Mass of hydrogen gas produced = (5 moles )× (2 g/mol)
Mass of hydrogen gas produced = 10 grams