# Moles and Grams - 5

#### TheJason

Staff member
Moderator
How many moles of $$\displaystyle H$$ are in $$\displaystyle 40,000 \,mg\,$$ of $$\displaystyle H_{2}\,O$$ - to three significant figures?

Find the molar mass (formula weight) of water.

For $$\displaystyle H_{2}\,O$$, finding via the periodic table (atomic weights):

H - $$\displaystyle 1.00794 (2) \,g\,\, H = 2.01588 \,g\,\, H$$

O - $$\displaystyle 15.9994 \,g\,\, O$$

$$\displaystyle 2.01588\,g\, \,H + 15.9994\,g\,\, O = 18.01528\,g\,\, H_{2}\,O$$

Final Calculation

$$\displaystyle 40,000\,mg\, H_{2}\,O\,(\dfrac{ 0.001\,g\, H_{2}\,O}{ 1\,mg\,\, H_{2}\,O}))(\dfrac{1\,mol\,H_{2}\,O}{18.01528\,g\,\,H_{2}\,O }(\dfrac{2\,mol\,H}{1\,mol\,H_{2}\,O}) = 4,44 \,mol\,H$$

How many moles of $$\displaystyle Cu$$ are in $$\displaystyle 100,000\,mg$$ of $$\displaystyle CuO$$ - to three significant figures?