How the cycling of matter and flow of energy change in this ecosystem

Why does his teacher ask him to balance the equation by including the correct coefficient

Solutions of sodium carbonate and silver nitrate react to form solid silver carbonate and a solution of sodium nitrate. a solution containing 3.50 g of sodium carbonate is mixed with one containing 5.00 g of silver nitrate. how many grams of sodium carbonate, silver nitrate, silver carbonate, and sodium nitrate are present after the reaction is complete?

Which property of a rock remains unchanged by mechanical weathering? a. total surface area b. size and shape c. mineral composition d. sharpness