For elements 1-20, we can use the periodic table to predict.

a) an elements color
b) the number of valence electrons an element has
c) the number of isotopes an element has.

B

Explanation:

because a periodic table shows the arrangement of element according to their atomic number and their valence neither

isotopes nor colour

B.

Explanation:

We can predict the number of valence electrons  by which group of the Periodic table an element resides.

For example Lithium is in group 1  and has 1 valency electron.

"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"