Answer the questions on a separate sheet of paper. You must write your name on t

Question

Answer the questions on a separate sheet of paper. You must write your name on the paper and date it. A 50.0 mL sample of a gas was found to have a pressure of 1.0 atm at 25 degree C. What will be the new volume if the pressure is changed to 0.50 atm at constant temperature? The temperature of20.0 mL of certain gas was changed from 25.0 degree C to 50 degree C at constant pressure. What is the new volume of the gas in mL? Calculate: (i) The number of atoms in 2.50 g of potassium (ii) The number of molecules in 3.00 g of nitrogen gas, N_2 A 1.00 L sample of chlorine gas at 25.0 degree C was found to have a pressure of 1.00 atm. How many moles of gas are present and what is the mass of the gas in grams? Calculate the volume of CO_2 gas at STP obtained by reacting a 10.00 g sample of calcium carbonate with an excess of hydrochloric acid: CaCO_3(s) + 2 HCl (aq) rightarrow CaCl_2(aq) + CO_2 (g) + H_2 O(l) A student prepared Na_2CO_3 by bubbling CO_2 through an aqueous solution of NaOH; 2 NaOH(aq) + CO_2 rightarrow Na_2CO_3 (aq) + H_2 O(l) If 15.0 g of NaOH was used, what mass of Na_2 CO_3 is expected?

Explanation / Answer

Ans1.

A 50 mL sample of a gas found to have a pressure of 1 atm at 25 0C. When the pressure will change to 0.50 atm at constant temperature, the change of volume can be calculated as follows

Initial volume of the sample Vl =50 mL,

Initial Pressure of the sample P1= 1atm

Initial Pressure of the sample P1= 0.50 atm

We have to calculate the final volume of the sample i.e V2

As Temperature is given constant, so we can apply Boyle’s law to calculate the final volume

(Boyle’s law states that: at const. temp, Pressure of a given mass of gas is inversely proportional to its volume i.e

P 1/V, hence PV= constant, so P1V1=P2V2)

P1V1=P2V2

V2= P1V1/P2= (1* 50)/ 0.5= 100 mL

Hence the gas will occupy a volume of 100 mL at a pressure of 0.5 atm

Ans.2.

The temperature of 20 mL of a certain gas changes from 25C to 50C. The new volume of the gas at constant temperature can be calculated as follows

Initial volume V1=20 mL, Initial temperature T1=25C +273=298K

Final temperature T2=50 C+273= 323K, so we have to calculate the final volume V2

As given, Volume is changing with change is temperature at constant pressure, we can apply Charles’ law (The law states that at constant pressure volume of a given mass of gas is directly proportional to its temperature. i.e V T, hence V/T=constant, i.e V1/T1=V2/T2)

So applying Charles law V1/T1=V2/T2

V2= (V1*T2)/T1= (20*323)/298= 21.67 mL

Hence at constant pressure, the gas will occupy a volume of 21.67 mL at 50 C

Ans3a.

Molar mass of Potassium (K) = 39.09 g/mol. i.e 39.09 gram of K is present in 1 mole.

We know that 1 mole of any substance contains Avogadro’s number of atoms of the same substance

So 39.09 g of K contain 6.023*1023 no. of K atoms

Hence, 2.5 g of K will contain = (6.023*1023* 2.5)/ 39.09= 0.3852*1023 atoms

Ans 3b

Molar mass of Nitrogen (N2) molecule =28g

As we know from Mole concept, gram molecular mass of a gas contain Avogadro number of molecules.

i.e 28 g of N2 gas contain  6.023*1023 no. of molecules

Hence 3g of N2 gas will contain= (6.023*1023 * 3)/28= 0.64532 *1023 no. of molecules of N2 gas

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