At which value of $pH$ of the inner side of the mouth does the decay of teeth take place?

$(a)$ Write the chemical name and chemical formula of washing soda.
$(b)$ How is it obtained from sodium chloride? Give equations of the reactions.
$(c)$ Why is it called a basic salt? Give any one of its uses.

Name the gas evolved when dilute $HCl$ reacts with sodium hydrogen carbonate. How is it recognized?

Which of the following formulae is correct for calculating the number of moles?

How is bleaching powder prepared? Why does bleaching powder $(i)$ not dissolve completely in water?

What is the $pH$ value of a basic aqueous solution?