[dylancampbell]

Okay I have been having a very hard time with these questions, which all have to do with chemical reactions dealing with molarities of substances.

1) Magnesium metal reacts with HCl solution, liberating H2 gas and generating Mg2+ cations in solution. A 1.030-g sample of Mg metal is added to 125.0 mL of a 4.00 M HCl solution, and the reaction goes to completion. How many g of H2 are formed?

2) Iron reacts with HCl solution to give H2 gas and Fe2+ ions. Suppose that 3.1 g of iron are to be dissolved in 1.8 M HCl. What is the minimum volume (in mL) of the acid solution required to react with all of the iron?

3) A sample is marked as containing 22.6 percent calcium carbonate by mass. How many grams of calcium carbonate are contained in 47.8 grams of the sample ?


The farthest I've gotten is calculating that I have .5 moles of HCl in the first problem, and I have no idea where to go from there. Any help would be greatly appreciated.

[Yggdrasil]

For 1 & 2, you should first write out a balanced chemical reaction described.

Number 3 is just asking what is 22.6% of 47.8g.

[Quantum07]

For number 1. This is a limiting reagent problem

Mg(s) + 2HCl ----> MgCl₂ + H₂

1.030g Mg / 24.312 g/mol Mg =   0.04236 moles Mg

(0.1250 L )x(4.00M HCl)=  0.500 moles HCl

According to the balanced equation for every 1 mol of Mg you need 2 moles of HCl. Since you have 0.04236 mol Mg you would need 0.08472 mols of HCl and you have 0.500 moles which is more than enough. This means you have excess HCl and the amount of H2 liberated will depend only on the amount of Mg you used so...to get the grams of H2 you would get.

0.04236 mols Mg x ( 1 mol H₂/1 mol Mg) x (2.01594 g H2 / 1 mol H₂)

gives you

0.08539 g H2