Question

9. In what year does the Russian chemist Dimitri Mendeleev started the
development of the periodic table, arranging chemical elements by atomic
mass?
A. 1866
B. 1867
C. 1868
D. 1869

10. How many elements are there in the formula: Ca(OH).
B. 3
C. 7
D. 8​

Answer 1

Answer:

a

9. In what year does the Russian chemist Dimitri Mendeleev started the

development of the periodic table, arranging chemical elements by atomic

mass?

A. 1866

B. 1867

C. 1868

D. 1869✓

10, this explanation maybe help you

Answer:

5

Explanation:

Ca is 1 since there is no subscript and O and H each have 2 because the subscript 2 is outside the parenthesis.

i. e. 1 Ca + 2 O + 2 H = 5 atoms

Hope this helps