What are the limitations of Mendeleev's periodic table?

Which of the following statement are incorrect regarding Mendeleev's periodic table?

(a) Mendeleev considered compounds formed by elements with oxygen and hydrogen.

(b) In the table Ni is placed before Co

(c) Eka-silicon in Mendeleev's periodic table is gallium

(d) The properties of the elements are periodic functions of their atomic masses.

Why do you think the noble gases are placed in a separate group? 

An element $\mathrm{X}$ form an oxide ${\mathrm{X}}_2{\mathrm{O}}_3$. In which group of Mendeleev's periodic table, is this element $\mathrm{X}$.

Give any two limitations of Mendeleev's periodic classification.

Write merits of Mendeleev's periodic table. 

Use Mendeleev's Periodic Table to predict the formulae for oxides of the following elements: 

$\mathrm{K}$ and $\mathrm{Al}$.