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March 31, 2009

Your exams are over and a new academic session has begun! So, Topper helps you gear up by answering ALL your queries, within 72 hours!

Here are four questions asked by Topper readers from Class IX, and the answers by Topper maths and science experts.

Subject: Chemistry

Q. Calculate the number of atoms in 0.07g of nitrogen gas.

Posted by: Iona Dias

A. 1 mole of N2 gas contains 2 x 6.023 x 1023 atoms of N because N2 gas is a diatomic molecule. 1 molecule of N2 gas is made up of 2 atoms of N

For 0.07 g of N2 gas, number of moles =0.07/28=2.5 x 10-3

If 1 mole of N2 gas contains 2.5 x 10-3 atoms of N

Therefore 2.5 x 10-3 mol of N2 gas contains 2.5 x 10-3 x 2 x 6.023 x 1023 =3.011x 1021  atoms of N.

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Subject: Physics

Q. The distance b/w Delhi and Agra is 200 m. A train travels the first 100 km at a speed of 50 km/hr. How fast must the train travel the next 100 km, so as to average 70 km/hr for the whole journey?

Posted by: JSharma

A. The average speed = total distance / total time
The average speed required = 70km/hr

Total distance = 200 km

Speed of the train for the first 100 km = 50 km/hr
Time taken= 100/50 = 2 hours

Let the speed for the next 100 km be x km/hr

Time taken = 100/x
The total time taken = 2 +100 / x

Hence 70 = 200 / [ 2+100/x]
2+ 100/x = 20/7
x = 116.67 km/hr

Hence, the train must move with a speed of 116.67 km/hr.