# Q.1. How many molecules are present in(a) 9 grams of water(b) 17 grams of amoniaQ.2. Calculate the number of moles in 17 grams of H2O2 Q.3. Show that 12 grams of carbon-12 contains 6.023 x 1023 atoms of 12CQ.4. What is the mass of one atom of hydrogen ?Q.5. How many moles of Mg atoms are there in 100 grams of magnesium ?Q.6. Neon gas consist of single atoms. What mass of neon contains 6.023 x 1023 atoms ?

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1) No. of molecules in 9g of water:

Molecular mass of water(H2O)= (1×2)+16=18g/mol

1 mole of water=18g of water contains=6.022×1023 molecules of water

then 9g of water will have= (6.022×1023/18)×9 = 3.011×1023 molecules

2) No. of moles in 17g of H2O2:

no. of moles= mass / molecular mass

Molecular mass of  H2O2= (1×2)+(16×2)=34g/mole

so no. of moles of  H2O2 in 17g= 17/34=0.5

4)Mass of one atom of hydrogen:

1 mole i.e. 6.022×1023 atoms of hydrogen= 1g

then mass of one atom of hydrogen= 1/(6.022×1023)=1.6×10-24 g

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Ans.1 (a) no. of molecules = moles * 6.023 * 1023 = (9/18)* 6.023 * 1023 = 3.011 * 1023

Ans.1 (b) no. of molecules = moles * 6.023 * 1023 = (17/17)* 6.023 * 1023 = 6.023 * 1023

Ans.2 = no. of moles = given mass / molecular mass = 17 / 34 = 0.5 moles

Ans.4 :- mass of 1 mole of H = 1g

mass of 6.023 * 1023 atoms og H = 1g

mass of 1 atom of H = 1 / 6.023 * 1023 g

Ans.5 :-  no. of moles = given mass / molecular mass = 100 / 22 = 50/11

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