some basic concepts of chemistry

Chlorine is prepared in the laboratory by treating manganese dioxide (MnO2) with aqueous hydrochloric acid according to the reaction:

4 HCl (aq) + MnO2(s) → 2H2O (l) + MnCl2(aq) + Cl2 (g)

How many grams of HCl react with 5.0 g of manganese dioxide?
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Calcium carbonate reacts with aqueous HCl to give CaCl2 and CO2 according to the reaction, CaCO3 (s) + 2 HCl (aq) →  CaCl2(aq) + CO2 (g) + H2O(l). What mass of CaCO3 is required to react completely with 25 mL of 0.75 M HCl?
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A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g.  Find:

(i) Empirical formula

(ii) Molar mass of the gas, and

(iii) Molecular formula
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Calculate the number of atoms in each of the following

(i) 52 moles of Ar

(ii) 52 u of He

(iii) 52 g of He
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Use the data given in the following table to calculate the molar mass of naturally occurring argon isotopes:

Isotope Molar mass Abundance
\(\, _{ 36 }\textrm{Ar}\) \( 35.96755 g \; mol^{ -1 }\) 0.337 %
\(\, _{ 38 }\textrm{Ar}\) \( 37.96272 g \; mol^{ -1 }\) 0.063 %
\(\, _{ 40 }\textrm{Ar}\) \( 39.9624g \; mol^{ -1 }\) 99.600 %
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