From 200mg of carbon dioxide, 10 to the power 21 molecules are removed. How many frams and moles of carbon dioxide are left?

First convert the mass of CO₂ into moles.

200 mg CO₂ x (1 g CO₂ / 1000 mg CO₂) = 0.200 g CO₂.

The molar mass of CO2 (found by adding up the atomic weights for CO2 from the periodic table) = C (1 x 12.0) + 2 O (2 x 16.0) = 44.0 g CO2 per mole of CO2.

0.200 g CO2 x (1 mole CO2 / 44.0 g CO2) = 0.00455 moles of CO2.

One mole of anything contains 6.02 x 10²³ particles (molecules or atoms).

0.00456 moles CO2 x (6.02 x 10²³ molecules CO2 / 1 mole CO2) = 2.74 x 10²¹ molecules CO2

So if you take away 1 x 10²¹ molecules, you have left

2.74 x 10²¹ - 1.0 x 10²¹ = 1.74 x 10²¹ molecules CO2