O2 means we have 0.25 moles of O2.  As, 1 MOLE has 6.023*10^23 molecules, 0.25 moles of O2 will have 0.25*6.023*10^23 molecules=1.50575*10^23 molecules and as 1 molecule of O2 has 2 atoms, so, 1.50575*10^23 molecules will have 2*1.50575*10^23 atoms=3.0115*10^23 atoms of O.Explanation:At NTP , one mole of a gas occupies a volume of 22.4 L . This particular volume is KNOWN as molar volume . Also we know that one mole of a gas contains avogadro number of molecules and it contains atoms EQUAL to the PRODUCT of number of molecules and atomicity . The atomicity of oxygen is 2 , so 1 mole of oxygen contains atoms equal to 2 times the number of molecules Therefore we can SAY that 22.4 L of Oxygen contains = 6.023 * 10^(23) molecules 1 L of Oxygen contains = 6.023 * 10^(23)/22.4 molecules 5.6 L of Oxygen contains = 6.023 * 10^(23) * 5.6/22.4 molecules 5.6 L of Oxygen contains = 6.023 * 10^(23)/4 molecules 5.6 L of Oxygen contains = 1.50575 * 10^(23) molecules Therefore , 5.6 L of Oxygen contains = 1.50575 * 10^(23) * 2 atoms 5.6 L of Oxygen contains = 3.0115 * 10^(23) atomsHope it helps....please mark as brainliest.