The reactivity of an alkali metal depends on the metals ability to

             The reactivity of an alkali metal depends on the metals ability to loose an electron.

                         Alkali metals are elements present in first group of the periodic table. Following are the metals from top to bottom;

                                               Lithium

                                               Sodium

                                               Potassium

                                               Rubidium

                                               Cesium

                                               Francium

These s-block elements are considered the most reactive metals in the periodic table because they have only one electron in their valence shell and readily loose that electron to form an ionic bond with non metal. The general electronic configuration of these metals is as follow,

                                                       nS¹

Also, the reactivity of these metals increases from top to bottom in the group as the ionization energy decreases due to increase in shielding effect respectively.

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2