Find the number of moles of H2 needed to produce 27 grams of HCl.
1H2 + 1Cl2 ➡️ 2HCl

Mendeleev placed thallium (Tl) in the same group as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs)

Explanation:

Properties of Thallium :

Thallium is a heavy metal denoted by symbol Tl. It is placed in 13th group and 6th period of modern periodic table. It is very soft in nature. It has 3 valency. It does not occur freely in environment.

Mendeleev  grouped thallium in the same group as lithium and sodium because of he found that as Li, Na etc. have compound with one atom of chlorine(Cl), Thallium(Tl) also forms TlCl. Therefore he placed Tl with these elements.

Thallium is different from the other five elements

Tl has 3 valency whereas lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) has only 1 valency.